Report errors on repeated interfaces.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 2762 matching lines @@
       DartType interfaceType = typeResolver.resolveTypeAnnotation(
           link.head, scope, element, onFailure: error);
       if (interfaceType != null) {
         if (identical(interfaceType.kind, TypeKind.MALFORMED_TYPE)) {
           // Error has already been reported.
         } else if (!identical(interfaceType.kind, TypeKind.INTERFACE)) {
           // TODO(johnniwinther): Handle dynamic.
           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]);
           }
         }
       }
     }
     element.interfaces = interfaces;
     calculateAllSupertypes(element);
 
@@ skipping 596 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }