Report errors on repeated interfaces.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 101617187f15ab9e4be91ccd433c77f7a72f3314..245a2e4bc3b5622dd33079ec705a8c24afe6ef60 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -2780,6 +2780,22 @@ class ClassResolverVisitor extends TypeDefinitionVisitor {
           TypeAnnotation typeAnnotation = link.head;
           error(typeAnnotation.typeName, MessageKind.CLASS_NAME_EXPECTED, []);
         } else {
+          if (interfaceType == element.supertype) {
+            compiler.reportMessage(
+                compiler.spanFromNode(node.superclass),
+                MessageKind.DUPLICATE_EXTENDS_IMPLEMENTS.error([interfaceType]),
+                Diagnostic.ERROR);
+            compiler.reportMessage(
+                compiler.spanFromNode(link.head),
+                MessageKind.DUPLICATE_EXTENDS_IMPLEMENTS.error([interfaceType]),
+                Diagnostic.ERROR);
+          }
+          if (interfaces.contains(interfaceType)) {
+            compiler.reportMessage(
+                compiler.spanFromNode(link.head),
+                MessageKind.DUPLICATE_IMPLEMENTS.error([interfaceType]),
+                Diagnostic.ERROR);
+          }
           interfaces = interfaces.prepend(interfaceType);
           if (isBlackListed(interfaceType)) {
             error(link.head, MessageKind.CANNOT_IMPLEMENT, [interfaceType]);