Report errors on duplicate export.

tests/language/duplicate_import_libd.dart

Index: tests/language/duplicate_import_libd.dart
diff --git a/tests/language/duplicate_import_libd.dart b/tests/language/duplicate_import_libd.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ccec81e0ff6df566051ae1aa52b7e8c4e6c6fc48
--- /dev/null
+++ b/tests/language/duplicate_import_libd.dart
@@ -0,0 +1,13 @@
+// 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.
+
+library libd;
+
+import 'duplicate_import_liba.dart';
+import 'duplicate_import_libb.dart';
+import 'duplicate_import_libc.dart';
+
+export 'duplicate_import_liba.dart';
+export 'duplicate_import_libb.dart';
+export 'duplicate_import_libc.dart';