If you’re fiddling around with django and advanced admin interfaces it can happen that you get an error like:
Model foobar is already registered at …
with an link to …/contrib/admin.py.

It took me a while to figure out what happened, however, the solution is to import all related models projectlike. So that you have import statements like that:
from project.app.models import Class.
The same is valid for the settings.py Apps section.