`installed_app_names` contains the names of the apps, which is not necessarily what is in `INSTALLED_APPS`. Because we now have `django.contrib.admin.apps.PlainAdminConfig`, and we look at contrib for places to find tests and pick up `django.contrib.admin` this resulted in trying to add it twice, and thus (in the old version) raising a warning. Whether this is still a problem is an interesting question, but it seemed wrong to me to be re-adding `django.contrib.admin` with a default AppConfig. As @aaugustin knows more about this I'd like him to look at the change