Fixed #7885 -- Prevent duplicates in fields and fieldsets declarations. Thanks...

Fixed #7885 -- Prevent duplicates in fields and fieldsets declarations. Thanks julien and wamberg for the patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8173 bcc190cf-cafb-0310-a4f2-bffc1f526a37