Process each namespace only once while reporting redeclaration errors

compiler/frontend/src/org/jetbrains/jet/lang/resolve/DeclarationResolver.java

@@ -351,7 +351,7 @@ public class DeclarationResolver {
     }
 
     private void checkRedeclarationsInNamespaces() {
-        for (NamespaceDescriptorImpl descriptor : context.getNamespaceDescriptors().values()) {
+        for (NamespaceDescriptorImpl descriptor : Sets.newHashSet(context.getNamespaceDescriptors().values())) {
             Multimap<Name, DeclarationDescriptor> simpleNameDescriptors = descriptor.getMemberScope().getDeclaredDescriptorsAccessibleBySimpleName();
             for (Name name : simpleNameDescriptors.keySet()) {
                 // Keep only properties with no receiver

compiler/testData/diagnostics/tests/redeclarations/RedeclarationInMultiFile.kt

@@ -0,0 +1,5 @@
+// FILE: a.kt
+val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+
+// FILE: b.kt
+val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -4650,6 +4650,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/redeclarations/PropertyAndInnerClass.kt");
             }
             
+            @TestMetadata("RedeclarationInMultiFile.kt")
+            public void testRedeclarationInMultiFile() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/redeclarations/RedeclarationInMultiFile.kt");
+            }
+            
             @TestMetadata("Redeclarations.kt")
             public void testRedeclarations() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/redeclarations/Redeclarations.kt");