12288 Added new check on INSTALL_APPS for unique values

https://code.djangoproject.com/ticket/12288 Removing lines 110-111 in django/django/conf/__init__.py causes the test to fail, but putting in the lines 110-111 allow tests to succeed. I've confirmed that the test works. In addition, the full Django test suite all passed. Thanks  @timgraham @jdunck @charettes for the advice and code review. 

 I'll appreciate feedback; feel free to code review. Thanks.