Fixed #13902 -- Fixed admin list_filter so it doesn't show duplicate results...

Fixed #13902 -- Fixed admin list_filter so it doesn't show duplicate results when it includes a field spec that involves m2m relationships with an intermediate model. Thanks Iván Raskovsky for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15526 bcc190cf-cafb-0310-a4f2-bffc1f526a37