diff --git a/compiler/frontend/src/org/jetbrains/kotlin/cfg/ControlFlowProcessor.kt b/compiler/frontend/src/org/jetbrains/kotlin/cfg/ControlFlowProcessor.kt
index b8de945f29c..0280cff57ba 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/cfg/ControlFlowProcessor.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/cfg/ControlFlowProcessor.kt
@@ -1202,6 +1202,7 @@ class ControlFlowProcessor(private val trace: BindingTrace) {
             builder.bindLabel(doneLabel)
 
             mergeValues(branches, expression)
+            WhenChecker.checkDuplicatedLabels(expression, trace)
         }
 
         override fun visitObjectLiteralExpression(expression: KtObjectLiteralExpression) {
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.kt b/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.kt
index 33e7948d9fd..fb6b5c878ef 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.kt
@@ -37,6 +37,8 @@ import org.jetbrains.kotlin.types.TypeUtils
 import org.jetbrains.kotlin.resolve.DescriptorUtils.isEnumClass
 import org.jetbrains.kotlin.resolve.DescriptorUtils.isEnumEntry
 import org.jetbrains.kotlin.resolve.bindingContextUtil.isUsedAsExpression
+import org.jetbrains.kotlin.resolve.constants.CompileTimeConstant
+import org.jetbrains.kotlin.resolve.constants.evaluate.ConstantExpressionEvaluator
 import java.util.*
 
 interface WhenMissingCase {
@@ -333,6 +335,46 @@ object WhenChecker {
     fun containsNullCase(expression: KtWhenExpression, context: BindingContext) =
             WhenOnNullableExhaustivenessChecker.getMissingCases(expression, context, true).isEmpty()
 
+    fun checkDuplicatedLabels(expression: KtWhenExpression, trace: BindingTrace) {
+        if (expression.subjectExpression == null) return
+
+        val checkedTypes = HashSet<Pair<KotlinType, Boolean>>()
+        val checkedConstants = HashSet<CompileTimeConstant<*>>()
+        for (entry in expression.entries) {
+            if (entry.isElse) continue
+
+            conditions@ for (condition in entry.conditions) {
+                when (condition) {
+                    is KtWhenConditionWithExpression -> {
+                        val constantExpression = condition.expression ?: continue@conditions
+                        val constant = ConstantExpressionEvaluator.getConstant(
+                                constantExpression, trace.bindingContext) ?: continue@conditions
+                        if (checkedConstants.contains(constant)) {
+                            trace.report(Errors.DUPLICATE_LABEL_IN_WHEN.on(constantExpression))
+                        }
+                        else {
+                            checkedConstants.add(constant)
+                        }
+
+                    }
+                    is KtWhenConditionIsPattern -> {
+                        val typeReference = condition.typeReference ?: continue@conditions
+                        val type = trace.get(BindingContext.TYPE, typeReference) ?: continue@conditions
+                        val typeWithIsNegation = type to condition.isNegated
+                        if (checkedTypes.contains(typeWithIsNegation)) {
+                            trace.report(Errors.DUPLICATE_LABEL_IN_WHEN.on(typeReference))
+                        }
+                        else {
+                            checkedTypes.add(typeWithIsNegation)
+                        }
+                    }
+                    else -> {}
+                }
+            }
+        }
+
+    }
+
     fun checkDeprecatedWhenSyntax(trace: BindingTrace, expression: KtWhenExpression) {
         if (expression.subjectExpression != null) return
 
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
index 67a204f9c33..5ce5701c3b4 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
@@ -728,6 +728,7 @@ public interface Errors {
     DiagnosticFactory1<KtWhenExpression, List<WhenMissingCase>> NO_ELSE_IN_WHEN = DiagnosticFactory1.create(ERROR, WHEN_EXPRESSION);
     DiagnosticFactory1<KtWhenExpression, List<WhenMissingCase>> NON_EXHAUSTIVE_WHEN = DiagnosticFactory1.create(WARNING, WHEN_EXPRESSION);
     DiagnosticFactory0<PsiElement> COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<PsiElement> DUPLICATE_LABEL_IN_WHEN = DiagnosticFactory0.create(WARNING);
 
     // Type mismatch
 
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
index f769c629113..f066433affe 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
@@ -430,6 +430,7 @@ public class DefaultErrorMessages {
 
         MAP.put(ELSE_MISPLACED_IN_WHEN, "'else' entry must be the last one in a when-expression");
         MAP.put(COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT, "Deprecated syntax. Use '||' instead of commas in when-condition for 'when' without argument");
+        MAP.put(DUPLICATE_LABEL_IN_WHEN, "Duplicate label in when");
 
         MAP.put(NO_ELSE_IN_WHEN, "''when'' expression must be exhaustive, add necessary {0}", RENDER_WHEN_MISSING_CASES);
         MAP.put(NON_EXHAUSTIVE_WHEN, "''when'' expression on enum is recommended to be exhaustive, add {0}", RENDER_WHEN_MISSING_CASES);
diff --git a/compiler/testData/diagnostics/tests/generics/RawTypeInIsPattern.kt b/compiler/testData/diagnostics/tests/generics/RawTypeInIsPattern.kt
index 6decc0f4180..e62b26b7934 100644
--- a/compiler/testData/diagnostics/tests/generics/RawTypeInIsPattern.kt
+++ b/compiler/testData/diagnostics/tests/generics/RawTypeInIsPattern.kt
@@ -3,7 +3,7 @@ public fun foo(a: Any, <!UNUSED_PARAMETER!>b<!>: <!WRONG_NUMBER_OF_TYPE_ARGUMENT
         is Map<!WRONG_NUMBER_OF_TYPE_ARGUMENTS!><Int><!> -> {}
         is <!NO_TYPE_ARGUMENTS_ON_RHS!>Map<!> -> {}
         is Map<out Any?, Any?> -> {}
-        is Map<*, *> -> {}
+        is <!DUPLICATE_LABEL_IN_WHEN!>Map<*, *><!> -> {}
         is Map<<!SYNTAX!><!>> -> {}
         is List<<!WRONG_NUMBER_OF_TYPE_ARGUMENTS!>Map<!>> -> {}
         is <!NO_TYPE_ARGUMENTS_ON_RHS!>List<!> -> {}
diff --git a/compiler/testData/diagnostics/tests/when/DuplicatedLabels.kt b/compiler/testData/diagnostics/tests/when/DuplicatedLabels.kt
new file mode 100644
index 00000000000..4661ede0dea
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/DuplicatedLabels.kt
@@ -0,0 +1,51 @@
+package test
+
+const val four = 4
+
+fun first(arg: Int) = when (arg) {
+    1 -> 2
+    2 -> 3
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 4
+    4 -> 5
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 6
+    <!DUPLICATE_LABEL_IN_WHEN!>2<!> -> 7
+    // Error should be here: see KT-11971
+    four -> 8
+    else -> 0
+}
+
+fun second(arg: String): Int {
+    when (arg) {
+        "ABC" -> return 0
+        "DEF" -> return 1
+        <!DUPLICATE_LABEL_IN_WHEN!>"ABC"<!> -> return -1
+        <!DUPLICATE_LABEL_IN_WHEN!>"DEF"<!> -> return -2
+    }
+    return 42
+}
+
+fun third(arg: Any?): Int {
+    when (arg) {
+        null -> return -1
+        is String -> return 0
+        is Double -> return 1
+        is <!DUPLICATE_LABEL_IN_WHEN!>Double<!> -> return 2
+        <!DUPLICATE_LABEL_IN_WHEN!>null<!> -> return 3
+        else -> return 5
+    }
+}
+
+enum class Color { RED, GREEN, BLUE }
+
+fun fourth(arg: Color) = when (arg) {
+    Color.RED -> "RED"
+    Color.GREEN -> "GREEN"
+    <!DUPLICATE_LABEL_IN_WHEN!>Color.RED<!> -> "BLUE"
+    Color.BLUE -> "BLUE"
+}
+
+fun fifth(arg: Any?) = when (arg) {
+    is Any -> "Any"
+    <!ELSE_MISPLACED_IN_WHEN!>else<!> -> ""
+    <!UNREACHABLE_CODE!>else -> null<!>
+}
diff --git a/compiler/testData/diagnostics/tests/when/DuplicatedLabels.txt b/compiler/testData/diagnostics/tests/when/DuplicatedLabels.txt
new file mode 100644
index 00000000000..2a678a62287
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/DuplicatedLabels.txt
@@ -0,0 +1,31 @@
+package
+
+package test {
+    public const val four: kotlin.Int = 4
+    public fun fifth(/*0*/ arg: kotlin.Any?): kotlin.String?
+    public fun first(/*0*/ arg: kotlin.Int): kotlin.Int
+    public fun fourth(/*0*/ arg: test.Color): kotlin.String
+    public fun second(/*0*/ arg: kotlin.String): kotlin.Int
+    public fun third(/*0*/ arg: kotlin.Any?): kotlin.Int
+
+    public final enum class Color : kotlin.Enum<test.Color> {
+        enum entry RED
+
+        enum entry GREEN
+
+        enum entry BLUE
+
+        private constructor Color()
+        public final override /*1*/ /*fake_override*/ val name: kotlin.String
+        public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+        protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: test.Color): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        // Static members
+        public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): test.Color
+        public final /*synthesized*/ fun values(): kotlin.Array<test.Color>
+    }
+}
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/dynamicCastTarget.kt b/compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/dynamicCastTarget.kt
index 07aaa59629a..13530d6ab61 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/dynamicCastTarget.kt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/dynamicCastTarget.kt
@@ -15,9 +15,9 @@ fun test(d: Any, dl: Collection<dynamic>) {
 
     when (d) {
         is <!DYNAMIC_NOT_ALLOWED!>dynamic<!> -> {}
-        is <!DYNAMIC_NOT_ALLOWED!>dynamic?<!> -> {}
+        is <!DYNAMIC_NOT_ALLOWED, DUPLICATE_LABEL_IN_WHEN!>dynamic?<!> -> {}
         !is <!DYNAMIC_NOT_ALLOWED!>dynamic<!> -> {}
-        !is <!DYNAMIC_NOT_ALLOWED!>dynamic?<!> -> {}
+        !is <!DYNAMIC_NOT_ALLOWED, DUPLICATE_LABEL_IN_WHEN!>dynamic?<!> -> {}
     }
 
     dl as List<dynamic>
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
index 4438f8570f9..15a93dc638e 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
@@ -19260,6 +19260,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("DuplicatedLabels.kt")
+            public void testDuplicatedLabels() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/DuplicatedLabels.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("ElseOnNullableEnum.kt")
             public void testElseOnNullableEnum() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt");
diff --git a/core/descriptors/src/org/jetbrains/kotlin/resolve/constants/CompileTimeConstant.kt b/core/descriptors/src/org/jetbrains/kotlin/resolve/constants/CompileTimeConstant.kt
index ac2f8655718..3f0d0c55f95 100644
--- a/core/descriptors/src/org/jetbrains/kotlin/resolve/constants/CompileTimeConstant.kt
+++ b/core/descriptors/src/org/jetbrains/kotlin/resolve/constants/CompileTimeConstant.kt
@@ -56,6 +56,21 @@ class TypedCompileTimeConstant<out T>(
     val type: KotlinType = constantValue.type
 
     override fun toConstantValue(expectedType: KotlinType): ConstantValue<T> = constantValue
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is TypedCompileTimeConstant<*>) return false
+        if (isError) return other.isError
+        if (other.isError) return false
+        return constantValue.value == other.constantValue.value && type == other.type
+    }
+
+    override fun hashCode(): Int {
+        if (isError) return 13
+        var result = constantValue.value?.hashCode() ?: 0
+        result = 31 * result + type.hashCode()
+        return result
+    }
 }
 
 class IntegerValueTypeConstant(
@@ -92,4 +107,8 @@ class IntegerValueTypeConstant(
     fun getType(expectedType: KotlinType): KotlinType = TypeUtils.getPrimitiveNumberType(typeConstructor, expectedType)
 
     override fun toString() = typeConstructor.toString()
+
+    override fun equals(other: Any?) = other is IntegerValueTypeConstant && value == other.value
+
+    override fun hashCode() = value.hashCode()
 }