Address Siva's comments

runtime/vm/parser.cc

 // 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.
 
 #include "vm/parser.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/compiler_stats.h"
@@ skipping 8179 matching lines @@
   const Array& imports = Array::Handle(prefix.imports());
   for (intptr_t i = 0; i < prefix.num_imports(); i++) {
     import ^= imports.At(i);
     resolved_obj = LookupNameInImport(import, name);
     if (!resolved_obj.IsNull()) {
       obj = resolved_obj.raw();
       const Library& lib = Library::Handle(import.library());
       if (first_lib_url.IsNull()) {
         first_lib_url = lib.url();
       } else {
-        ErrorMsg(ident_pos,
-                 "ambiguous reference: '%s.%s' is defined in '%s' and '%s'",
-                 String::Handle(prefix.name()).ToCString(),
-                 name.ToCString(),
-                 first_lib_url.ToCString(),
-                 String::Handle(lib.url()).ToCString());
+        // Found duplicate definition.
+        if (first_lib_url.raw() == lib.url()) {
+          ErrorMsg(ident_pos,
+                   "ambiguous reference: '%s.%s' is imported multiple times",
+                   String::Handle(prefix.name()).ToCString(),
+                   name.ToCString());
+        } else {
+          ErrorMsg(ident_pos,
+                   "ambiguous reference: '%s.%s' is defined in '%s' and '%s'",
+                   String::Handle(prefix.name()).ToCString(),
+                   name.ToCString(),
+                   first_lib_url.ToCString(),
+                   String::Handle(lib.url()).ToCString());
+        }
       }
     }
   }
   return obj.raw();
 }
 
 
 RawClass* Parser::ResolveClassInPrefixScope(intptr_t ident_pos,
                                             const LibraryPrefix& prefix,
                                             const String& name) {
@@ skipping 1436 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart