Unhelpful admin.py error message

[Derek Willis]

If you make a boneheaded move such as creating a ModelAdmin class for a model that you forgot to include in your admin.py's model import statement, you get an error. But it's not the error you might expect. What happens is you get the "model already registered exception", citing the first model registered using admin.site.register().

I spent awhile looking around my app trying to see if I had indeed registered the model in question more than once via imports, but couldn't find it. When I fixed my error in the import statement, everything was fine.

So definitely user error. But the error message Django displayed didn't help guide me to the problem.

[Julien Phalip]

That's pretty much the same symptomatic issue as in #8245.