[1.3.X] Fixed #15819 - Fixed 1.3 regression from r15526 causing duplicate...

[1.3.X] Fixed #15819 - Fixed 1.3 regression from r15526 causing duplicate search results in admin with search_fields traversing to non-M2M related models. Thanks to Adam Kochanowski for the report and Ryan Kaskel for the patch.

Backport of r16093 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16094 bcc190cf-cafb-0310-a4f2-bffc1f526a37