diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorEquivalenceForOverrides.kt b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorEquivalenceForOverrides.kt
new file mode 100644
index 00000000000..abc3b409498
--- /dev/null
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorEquivalenceForOverrides.kt
@@ -0,0 +1,111 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.jet.lang.resolve
+
+import org.jetbrains.jet.lang.descriptors.DeclarationDescriptor
+import org.jetbrains.jet.lang.descriptors.ClassDescriptor
+import org.jetbrains.jet.lang.descriptors.CallableMemberDescriptor
+import org.jetbrains.jet.lang.descriptors.Visibilities
+import org.jetbrains.jet.lang.descriptors.PackageFragmentDescriptor
+import org.jetbrains.jet.lang.descriptors.TypeParameterDescriptor
+import org.jetbrains.jet.lang.resolve.OverridingUtil.OverrideCompatibilityInfo
+import org.jetbrains.jet.lang.resolve
+
+object DescriptorEquivalenceForOverrides {
+
+    public fun areEquivalent(a: DeclarationDescriptor?, b: DeclarationDescriptor?): Boolean {
+        return when {
+            a is ClassDescriptor &&
+            b is ClassDescriptor -> areClassesEquivalent(a, b)
+
+            a is TypeParameterDescriptor &&
+            b is TypeParameterDescriptor -> areTypeParametersEquivalent(a, b)
+
+            a is CallableMemberDescriptor &&
+            b is CallableMemberDescriptor -> areCallableMemberDescriptorsEquivalent(a, b)
+
+            a is PackageFragmentDescriptor &&
+            b is PackageFragmentDescriptor -> (a).fqName == (b).fqName
+
+            else -> a == b
+        }
+    }
+
+    private fun areClassesEquivalent(a: ClassDescriptor, b: ClassDescriptor): Boolean {
+        // type constructors are compared by fqName
+        return a.getTypeConstructor() == b.getTypeConstructor()
+    }
+
+    private fun areTypeParametersEquivalent(
+            a: TypeParameterDescriptor,
+            b: TypeParameterDescriptor,
+            equivalentCallables: (DeclarationDescriptor?, DeclarationDescriptor?) -> Boolean = {x, y -> false}
+    ): Boolean {
+        if (a == b) return true
+        if (a.getContainingDeclaration() == b.getContainingDeclaration()) return false
+
+        if (!ownersEquivalent(a, b, equivalentCallables)) return false
+
+        return a.getIndex() == b.getIndex() // We ignore type parameter names
+    }
+
+    private fun areCallableMemberDescriptorsEquivalent(a: CallableMemberDescriptor, b: CallableMemberDescriptor): Boolean {
+        if (a == b) return true
+        if (a.getName() != b.getName()) return false
+        if (a.getContainingDeclaration() == b.getContainingDeclaration()) return false
+
+        // Distinct locals are not equivalent
+        if (DescriptorUtils.isLocal(a) || DescriptorUtils.isLocal(b)) return false
+
+        if (!ownersEquivalent(a, b, {x, y -> false})) return false
+
+        val overridingUtil = OverridingUtil.createWithEqualityAxioms @eq {
+            (c1, c2): Boolean ->
+            if (c1 == c2) return@eq true
+
+            val d1 = c1.getDeclarationDescriptor()
+            val d2 = c2.getDeclarationDescriptor()
+
+            if (d1 !is TypeParameterDescriptor || d2 !is TypeParameterDescriptor) return@eq false
+
+            areTypeParametersEquivalent(d1, d2, {x, y -> x == a && y == b})
+        }
+
+        return overridingUtil.isOverridableBy(a, b).getResult() == OverrideCompatibilityInfo.Result.OVERRIDABLE
+                && overridingUtil.isOverridableBy(b, a).getResult() == OverrideCompatibilityInfo.Result.OVERRIDABLE
+
+    }
+
+    private fun ownersEquivalent(
+            a: DeclarationDescriptor,
+            b: DeclarationDescriptor,
+            equivalentCallables: (DeclarationDescriptor?, DeclarationDescriptor?) -> Boolean
+    ): Boolean {
+        val aOwner = a.getContainingDeclaration()
+        val bOwner = b.getContainingDeclaration()
+
+        // This check is needed when we call areTypeParametersEquivalent() from areCallableMemberDescriptorsEquivalent:
+        // if the type parameter owners are, e.g.,  functions, we'll go into infinite recursion here
+        if (aOwner is CallableMemberDescriptor || bOwner is CallableMemberDescriptor) {
+            return equivalentCallables(aOwner, bOwner)
+        }
+        else {
+            return areEquivalent(aOwner, bOwner)
+        }
+    }
+
+}
\ No newline at end of file
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/OverrideResolver.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/OverrideResolver.java
index b511a33ea7c..cc151f2f595 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/OverrideResolver.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/OverrideResolver.java
@@ -24,6 +24,7 @@ import com.intellij.util.Function;
 import com.intellij.util.containers.ContainerUtil;
 import com.intellij.util.containers.LinkedMultiMap;
 import com.intellij.util.containers.MultiMap;
+import com.intellij.util.containers.hash.EqualityPolicy;
 import kotlin.Function1;
 import kotlin.Unit;
 import org.jetbrains.annotations.NotNull;
@@ -40,6 +41,7 @@ import org.jetbrains.jet.lang.types.*;
 import org.jetbrains.jet.lang.types.checker.JetTypeChecker;
 import org.jetbrains.jet.lang.types.lang.KotlinBuiltIns;
 import org.jetbrains.jet.lexer.JetTokens;
+import org.jetbrains.jet.utils.HashSetUtil;
 
 import javax.inject.Inject;
 import java.util.*;
@@ -225,14 +227,37 @@ public class OverrideResolver {
     @NotNull
     private static <D> Set<D> filterOverrides(
             @NotNull Set<D> candidateSet,
-            @NotNull Function<? super D, ? extends CallableDescriptor> transform,
+            @NotNull final Function<? super D, ? extends CallableDescriptor> transform,
             @NotNull Filtering filtering
     ) {
+        if (candidateSet.size() <= 1) return candidateSet;
+
+        // In a multi-module project different "copies" of the same class may be present in different libraries,
+        // that's why we use structural equivalence for members (DescriptorEquivalenceForOverrides).
+        // Here we filter out structurally equivalent descriptors before processing overrides, because such descriptors
+        // "override" each other (overrides(f, g) = overrides(g, f) = true) and the code below removes them all from the
+        // candidates, unless we first compute noDuplicates
+        Set<D> noDuplicates = HashSetUtil.linkedHashSet(
+                candidateSet,
+                new EqualityPolicy<D>() {
+                    @Override
+                    public int getHashCode(D d) {
+                        return DescriptorUtils.getFqName(transform.fun(d).getContainingDeclaration()).hashCode();
+                    }
+
+                    @Override
+                    public boolean isEqual(D d1, D d2) {
+                        CallableDescriptor f = transform.fun(d1);
+                        CallableDescriptor g = transform.fun(d2);
+                        return DescriptorEquivalenceForOverrides.instance$.areEquivalent(f.getOriginal(), g.getOriginal());
+                    }
+                });
+
         Set<D> candidates = Sets.newLinkedHashSet();
         outerLoop:
-        for (D meD : candidateSet) {
+        for (D meD : noDuplicates) {
             CallableDescriptor me = transform.fun(meD);
-            for (D otherD : candidateSet) {
+            for (D otherD : noDuplicates) {
                 CallableDescriptor other = transform.fun(otherD);
                 if (me == other) continue;
                 if (filtering == Filtering.RETAIN_OVERRIDING) {
@@ -259,13 +284,22 @@ public class OverrideResolver {
             }
             candidates.add(meD);
         }
+
+        assert !candidates.isEmpty() : "All candidates filtered out from " + candidateSet;
+
         return candidates;
     }
 
+    // check whether f overrides g
     public static <D extends CallableDescriptor> boolean overrides(@NotNull D f, @NotNull D g) {
+        // This first check cover the case of duplicate classes in different modules:
+        // when B is defined in modules m1 and m2, and C (indirectly) inherits from both versions,
+        // we'll be getting sets of members that do not override each other, but are structurally equivalent.
+        // As other code relies on no equal descriptors passed here, we guard against f == g, but this may not be necessary
+        if (!f.equals(g) && DescriptorEquivalenceForOverrides.instance$.areEquivalent(f.getOriginal(), g.getOriginal())) return true;
         CallableDescriptor originalG = g.getOriginal();
         for (D overriddenFunction : getAllOverriddenDescriptors(f)) {
-            if (originalG.equals(overriddenFunction.getOriginal())) return true;
+            if (DescriptorEquivalenceForOverrides.instance$.areEquivalent(originalG, overriddenFunction.getOriginal())) return true;
         }
         return false;
     }
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParams.kt
new file mode 100644
index 00000000000..5d239df76e7
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParams.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T> {
+    public fun foo(a: T)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C<X> : B<X> {
+    override fun foo(a: X)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B<T> {
+    public fun foo(a: T)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B<String>?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo("")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsBoundMismatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsBoundMismatch.kt
new file mode 100644
index 00000000000..538eec35c9d
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsBoundMismatch.kt
@@ -0,0 +1,38 @@
+// !DIAGNOSTICS: -BASE_WITH_NULLABLE_UPPER_BOUND -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T> {
+    public fun foo(a: T?)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C<X> : B<X> {
+    override fun foo(a: X?)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait Tr
+
+public trait B<T: Tr?> {
+    public fun foo(a: T?)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B<Tr>?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo(null)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.kt
new file mode 100644
index 00000000000..bd30e5bc977
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -BASE_WITH_NULLABLE_UPPER_BOUND
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T, _> {
+    public fun foo(a: T?)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C<X, _> : B<X, _> {
+    override fun foo(a: X?)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B<_, T> {
+    public fun foo(a: T?)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun <Y, Z> test(b: B<Y, Z>?) {
+    if (b is C<Y, Z>) {
+        b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>foo<!>(null)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsNameMismatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsNameMismatch.kt
new file mode 100644
index 00000000000..2478762997c
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsNameMismatch.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T> {
+    public fun foo(a: T)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C<X> : B<X> {
+    override fun foo(a: X)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B<T1> {
+    public fun foo(a: T1)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun <Y> test(b: B<Y>?, y: Y) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo(y)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classVsFunctionGenericsInParamsMismatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classVsFunctionGenericsInParamsMismatch.kt
new file mode 100644
index 00000000000..46478cdea13
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classVsFunctionGenericsInParamsMismatch.kt
@@ -0,0 +1,36 @@
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T> {
+    public fun <R> foo(a: T, b : R)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B<Any?> {
+    override fun <R> foo(a: Any?, b : R)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun <T, R> foo(a: T, b: R)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?, c: C) {
+    b?.foo(1, 1)
+    c.foo(1, 1)
+    if (b is C) {
+        b?.<!CANNOT_COMPLETE_RESOLVE!>foo<!>(1, 1)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differenceInParamNames.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differenceInParamNames.kt
new file mode 100644
index 00000000000..af81b9ffb8a
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differenceInParamNames.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun foo(a: Int, b: String): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun foo(a: Int, b: String): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a1: Int, b1: String): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo(1, "")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentGenericsInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentGenericsInParams.kt
new file mode 100644
index 00000000000..dd765ec7db5
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentGenericsInParams.kt
@@ -0,0 +1,43 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m0
+// FILE: a.kt
+package p
+
+public trait G1<T>
+public trait G2<A, B>
+
+// MODULE: m1(m0)
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun foo(a: G1<Int>?, b: G2<B, String>?)
+}
+
+// MODULE: m2(m1, m0)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun foo(a: G1<Int>?, b: G2<B, String>?)
+
+}
+
+// MODULE: m3(m0)
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a: G1<out Any?>?, b: G2<Int, out Any?>?)
+}
+
+// MODULE: m4(m3, m2, m0)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>foo<!>(null, null)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentNumberOfParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentNumberOfParams.kt
new file mode 100644
index 00000000000..6384bf6263c
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentNumberOfParams.kt
@@ -0,0 +1,34 @@
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun foo(a: Int, b: String): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun foo(a: Int, b: String): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a: Int, b: String, c: Int = 0): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>foo<!>(1, "")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentReturnTypes.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentReturnTypes.kt
new file mode 100644
index 00000000000..d8fa721ba3d
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentReturnTypes.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun getParent(): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun getParent(): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun getParent(): Int
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.getParent()
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/extensionMatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/extensionMatch.kt
new file mode 100644
index 00000000000..1aa5dc3c464
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/extensionMatch.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun String.getParent(): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public class C : B {
+    override fun String.getParent(): B? = null
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun String.getParent(): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun B.test() {
+    if (this is C) {
+        "".getParent()
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParams.kt
new file mode 100644
index 00000000000..c536d7c9f53
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParams.kt
@@ -0,0 +1,42 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun <T> foo(a: T): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo<String>("")
+    }
+}
+
+fun test1(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo("")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsBoundsMismatch.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsBoundsMismatch.kt
new file mode 100644
index 00000000000..11e29337334
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsBoundsMismatch.kt
@@ -0,0 +1,40 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun <T> foo(a: T): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait Tr
+
+public trait B {
+    public fun <T: Tr?> foo(a: T): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        // hard to find parameters for an ambiguous call, so we rely on NONE_APPLICABLE here
+        // as opposed to diagnostics for a single unmatched candidate
+        b?.<!NONE_APPLICABLE!>foo<!>()
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsEqNull.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsEqNull.kt
new file mode 100644
index 00000000000..fd4333fa1f0
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsEqNull.kt
@@ -0,0 +1,52 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun <T> foo(a: T): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b == null) return
+    b?.foo<String>("")
+}
+
+fun test1(b: B?) {
+    if (b != null) {
+        b?.foo<String>("")
+    }
+}
+
+fun test2(b: B?) {
+    if (b == null) return
+    b?.foo("")
+}
+
+fun test3(b: B?) {
+    if (b != null) {
+        b?.foo("")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsNotIs.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsNotIs.kt
new file mode 100644
index 00000000000..6a8340a00aa
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsNotIs.kt
@@ -0,0 +1,40 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun <T> foo(a: T): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun <T> foo(a: T): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b !is C)  return
+    <!DEBUG_INFO_AUTOCAST!>b<!>?.foo("")
+}
+
+fun test1(b: B?) {
+    if (b !is C)  return
+    <!DEBUG_INFO_AUTOCAST!>b<!>?.foo<String>("")
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noGenericsInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noGenericsInParams.kt
new file mode 100644
index 00000000000..f76da0295a7
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noGenericsInParams.kt
@@ -0,0 +1,35 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun foo(a: Int, b: String): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun foo(a: Int, b: String): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a: Int, b: String): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo(1, "")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noParams.kt
new file mode 100644
index 00000000000..ef8ad5e5419
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noParams.kt
@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun getParent(): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public class C : B {
+    override fun getParent(): B? = null
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun getParent(): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.getParent()
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/sameGenericsInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/sameGenericsInParams.kt
new file mode 100644
index 00000000000..00d6327e6a9
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/sameGenericsInParams.kt
@@ -0,0 +1,43 @@
+// !DIAGNOSTICS: -UNNECESSARY_SAFE_CALL
+
+// MODULE: m0
+// FILE: a.kt
+package p
+
+public trait G1<T>
+public trait G2<A, B>
+
+// MODULE: m1(m0)
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun foo(a: G1<Int>, b: G2<B, String>): B?
+}
+
+// MODULE: m2(m1, m0)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun foo(a: G1<Int>, b: G2<B, String>): B?
+
+}
+
+// MODULE: m3(m0)
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a: G1<Int>, b: G2<B, String>): B?
+}
+
+// MODULE: m4(m3, m2, m0)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?, a: G1<Int>, b1: G2<B, String>) {
+    if (b is C) {
+        <!DEBUG_INFO_AUTOCAST!>b<!>?.foo(a, b1)
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/simpleWithInheritance.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/simpleWithInheritance.kt
new file mode 100644
index 00000000000..3b744395658
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/simpleWithInheritance.kt
@@ -0,0 +1,38 @@
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B {
+    public fun getParent(): B?
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B {
+    override fun getParent(): B?
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun getParent(): B?
+}
+
+public trait D : B {
+    override fun getParent(): B?
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C && b is D) {
+        b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>getParent<!>()
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/substitutedGenericInParams.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/substitutedGenericInParams.kt
new file mode 100644
index 00000000000..1e1f7af281f
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateMethod/substitutedGenericInParams.kt
@@ -0,0 +1,34 @@
+// MODULE: m1
+// FILE: a.kt
+package p
+
+public trait B<T> {
+    public fun foo(a: T)
+}
+
+// MODULE: m2(m1)
+// FILE: b.kt
+package p
+
+public trait C : B<String> {
+    override fun foo(a: String)
+
+}
+
+// MODULE: m3
+// FILE: b.kt
+package p
+
+public trait B {
+    public fun foo(a: String)
+}
+
+// MODULE: m4(m3, m2)
+// FILE: c.kt
+import p.*
+
+fun test(b: B?) {
+    if (b is C) {
+        b?.<!OVERLOAD_RESOLUTION_AMBIGUITY!>foo<!>("")
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/differentSuperTraits.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/differentSuperTraits.kt
new file mode 100644
index 00000000000..e464dd3e5af
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/differentSuperTraits.kt
@@ -0,0 +1,31 @@
+// MODULE: m1
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun foo() {}
+}
+
+public trait A : Base {
+    override fun foo() {}
+}
+
+public trait C : A
+
+
+// MODULE: m2
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun foo() {}
+}
+
+public trait B : Base
+
+// MODULE: m3(m1, m2)
+// FILE: x.kt
+
+import p.*
+
+class Foo: C, B
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTrait.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTrait.kt
new file mode 100644
index 00000000000..beec4542a11
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTrait.kt
@@ -0,0 +1,26 @@
+// MODULE: m1
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun foo() {}
+}
+
+public trait A : Base
+
+// MODULE: m2
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun foo() {}
+}
+
+public trait B : Base
+
+// MODULE: m3(m1, m2)
+// FILE: x.kt
+
+import p.*
+
+class Foo: A, B
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitDifferentBounds.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitDifferentBounds.kt
new file mode 100644
index 00000000000..fe614275034
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitDifferentBounds.kt
@@ -0,0 +1,29 @@
+// MODULE: m1
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun <T> foo(t: Array<T>) {}
+}
+
+public trait A : Base
+
+// MODULE: m2
+// FILE: x.kt
+package p
+
+public trait Base {
+    public fun <T: Base> foo(t: Array<T>) {}
+}
+
+public trait B : Base
+
+// MODULE: m3(m1, m2)
+// FILE: x.kt
+
+import p.*
+
+class Foo: A, B {
+    override fun <T> foo(t: Array<T>) {}
+    override fun <T: Base> foo(t: Array<T>) {}
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitGenerics.kt b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitGenerics.kt
new file mode 100644
index 00000000000..ce74602efd8
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitGenerics.kt
@@ -0,0 +1,26 @@
+// MODULE: m1
+// FILE: x.kt
+package p
+
+public trait Base<T> {
+    public fun foo(t: T) {}
+}
+
+public trait A<T> : Base<T>
+
+// MODULE: m2
+// FILE: x.kt
+package p
+
+public trait Base<T> {
+    public fun foo(t: T) {}
+}
+
+public trait B : Base<String>
+
+// MODULE: m3(m1, m2)
+// FILE: x.kt
+
+import p.*
+
+class Foo: A<String>, B
\ No newline at end of file
diff --git a/compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java
index 91c77f1c4e5..3b1ba774e8d 100644
--- a/compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java
@@ -4687,7 +4687,7 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
         }
         
         @TestMetadata("compiler/testData/diagnostics/tests/multimodule")
-        @InnerTestClasses({Multimodule.DuplicateClass.class})
+        @InnerTestClasses({Multimodule.DuplicateClass.class, Multimodule.DuplicateMethod.class, Multimodule.DuplicateSuper.class})
         public static class Multimodule extends AbstractJetDiagnosticsTest {
             public void testAllFilesPresentInMultimodule() throws Exception {
                 JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.TestsPackage", new File("compiler/testData/diagnostics/tests/multimodule"), Pattern.compile("^(.+)\\.kt$"), true);
@@ -4771,10 +4771,143 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 
             }
             
+            @TestMetadata("compiler/testData/diagnostics/tests/multimodule/duplicateMethod")
+            public static class DuplicateMethod extends AbstractJetDiagnosticsTest {
+                public void testAllFilesPresentInDuplicateMethod() throws Exception {
+                    JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.TestsPackage", new File("compiler/testData/diagnostics/tests/multimodule/duplicateMethod"), Pattern.compile("^(.+)\\.kt$"), true);
+                }
+                
+                @TestMetadata("classGenericsInParams.kt")
+                public void testClassGenericsInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParams.kt");
+                }
+                
+                @TestMetadata("classGenericsInParamsBoundMismatch.kt")
+                public void testClassGenericsInParamsBoundMismatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsBoundMismatch.kt");
+                }
+                
+                @TestMetadata("classGenericsInParamsIndexMismatch.kt")
+                public void testClassGenericsInParamsIndexMismatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.kt");
+                }
+                
+                @TestMetadata("classGenericsInParamsNameMismatch.kt")
+                public void testClassGenericsInParamsNameMismatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsNameMismatch.kt");
+                }
+                
+                @TestMetadata("classVsFunctionGenericsInParamsMismatch.kt")
+                public void testClassVsFunctionGenericsInParamsMismatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classVsFunctionGenericsInParamsMismatch.kt");
+                }
+                
+                @TestMetadata("differenceInParamNames.kt")
+                public void testDifferenceInParamNames() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differenceInParamNames.kt");
+                }
+                
+                @TestMetadata("differentGenericsInParams.kt")
+                public void testDifferentGenericsInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentGenericsInParams.kt");
+                }
+                
+                @TestMetadata("differentNumberOfParams.kt")
+                public void testDifferentNumberOfParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentNumberOfParams.kt");
+                }
+                
+                @TestMetadata("differentReturnTypes.kt")
+                public void testDifferentReturnTypes() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentReturnTypes.kt");
+                }
+                
+                @TestMetadata("extensionMatch.kt")
+                public void testExtensionMatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/extensionMatch.kt");
+                }
+                
+                @TestMetadata("functionGenericsInParams.kt")
+                public void testFunctionGenericsInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParams.kt");
+                }
+                
+                @TestMetadata("functionGenericsInParamsBoundsMismatch.kt")
+                public void testFunctionGenericsInParamsBoundsMismatch() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsBoundsMismatch.kt");
+                }
+                
+                @TestMetadata("functionGenericsInParamsEqNull.kt")
+                public void testFunctionGenericsInParamsEqNull() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsEqNull.kt");
+                }
+                
+                @TestMetadata("functionGenericsInParamsNotIs.kt")
+                public void testFunctionGenericsInParamsNotIs() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/functionGenericsInParamsNotIs.kt");
+                }
+                
+                @TestMetadata("noGenericsInParams.kt")
+                public void testNoGenericsInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noGenericsInParams.kt");
+                }
+                
+                @TestMetadata("noParams.kt")
+                public void testNoParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/noParams.kt");
+                }
+                
+                @TestMetadata("sameGenericsInParams.kt")
+                public void testSameGenericsInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/sameGenericsInParams.kt");
+                }
+                
+                @TestMetadata("simpleWithInheritance.kt")
+                public void testSimpleWithInheritance() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/simpleWithInheritance.kt");
+                }
+                
+                @TestMetadata("substitutedGenericInParams.kt")
+                public void testSubstitutedGenericInParams() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateMethod/substitutedGenericInParams.kt");
+                }
+                
+            }
+            
+            @TestMetadata("compiler/testData/diagnostics/tests/multimodule/duplicateSuper")
+            public static class DuplicateSuper extends AbstractJetDiagnosticsTest {
+                public void testAllFilesPresentInDuplicateSuper() throws Exception {
+                    JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.TestsPackage", new File("compiler/testData/diagnostics/tests/multimodule/duplicateSuper"), Pattern.compile("^(.+)\\.kt$"), true);
+                }
+                
+                @TestMetadata("differentSuperTraits.kt")
+                public void testDifferentSuperTraits() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateSuper/differentSuperTraits.kt");
+                }
+                
+                @TestMetadata("sameSuperTrait.kt")
+                public void testSameSuperTrait() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTrait.kt");
+                }
+                
+                @TestMetadata("sameSuperTraitDifferentBounds.kt")
+                public void testSameSuperTraitDifferentBounds() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitDifferentBounds.kt");
+                }
+                
+                @TestMetadata("sameSuperTraitGenerics.kt")
+                public void testSameSuperTraitGenerics() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/multimodule/duplicateSuper/sameSuperTraitGenerics.kt");
+                }
+                
+            }
+            
             public static Test innerSuite() {
                 TestSuite suite = new TestSuite("Multimodule");
                 suite.addTestSuite(Multimodule.class);
                 suite.addTestSuite(DuplicateClass.class);
+                suite.addTestSuite(DuplicateMethod.class);
+                suite.addTestSuite(DuplicateSuper.class);
                 return suite;
             }
         }
diff --git a/compiler/util/src/org/jetbrains/jet/utils/HashSetUtil.java b/compiler/util/src/org/jetbrains/jet/utils/HashSetUtil.java
new file mode 100644
index 00000000000..853fa7e352f
--- /dev/null
+++ b/compiler/util/src/org/jetbrains/jet/utils/HashSetUtil.java
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.jet.utils;
+
+import com.intellij.util.containers.hash.EqualityPolicy;
+import com.intellij.util.containers.hash.LinkedHashMap;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Map;
+import java.util.Set;
+
+public class HashSetUtil {
+    @NotNull
+    public static <T> Set<T> linkedHashSet(@NotNull Set<T> set,  @NotNull EqualityPolicy<T> policy) {
+        // this implementation of LinkedHashMap doesn't admit nulls as values
+        Map<T, String> map = new LinkedHashMap<T, String>(policy);
+        for (T t : set) {
+            map.put(t, "");
+        }
+        return map.keySet();
+    }
+}