[1.1.X] Fixed #12879: Declaring the same JS file multiple times in a single...

[1.1.X] Fixed #12879: Declaring the same JS file multiple times in a single Media instance now only includes that file once. Backport of [12663] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12664 bcc190cf-cafb-0310-a4f2-bffc1f526a37