Prevent users from saving top level menus that have duplicate URLs. All top menus must have unique URLs. 

Previously duplicates would get silently removed/overwritten, which could lead to (small amounts of) data loss.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@950202 b8457f37-d9ea-0310-8a92-e5e31aec5664