Fixed #14678 -- Added validation to catch flatpages with the same URL on the same site. Thanks seler for the report, and joni, graham_king, and j4nu5 for work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16937 bcc190cf-cafb-0310-a4f2-bffc1f526a37