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
If I understand correctly your question, `fixture_dirs` has no duplicates at the `dirs.extend()` moment and no default paths, so there is no such possibility (I don't see this) between checks and return.