Fixed #23468 -- settings.FIXTURE_DIRS checks in loaddata command

If settings.FIXTURE_DIRS contains duplicates or default fixture
directory (app_name/fixtures), ImproperlyConfigured is raised.
https://code.djangoproject.com/ticket/23468
This isn't a serious issue such that it qualifies for a backport to 1.7.x. See our [supported versions policy](https://docs.djangoproject.com/en/dev/internals/release-process/#supported-versions).