[1.2.X] Fixed #11707 - limit_choices_to on a ForeignKey can render duplicate options in formfield

Thanks to Chris Wesseling for the report and patch.

Backport of [15607] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15610 bcc190cf-cafb-0310-a4f2-bffc1f526a37