Simplify Apps.register_model

[Aymeric Augustin]

This method checks for the case when the same model is imported through two different Python paths. This check has lost its purpose with the new project layout introduced in Django 1.4.

Furthermore, if the check fails, the second model replaces the first one in the app registry. This behavior seems error-prone. Having two models with the same name in the same app should be an error.

In my opinion:

• the check for identity should be removed, most likely with a deprecation path for people who haven't switched to the new project layout yet.
• register_models should raise an exception when attempting to register two models with the same name.

[Aymeric Augustin]

Pull request: ​https://github.com/django/django/pull/2121

[Aymeric Augustin]

Considering the train wreck that was #1796, it isn't too surprising that some code was accidentally left in even though it couldn't be encountered.

This patch is quite safe: at worst, people will hit a RuntimeError at import time, which is obviously a blocker but not as perverse as app registry corruption.

[Aymeric Augustin]

Merged in aff57793b46e108c6e48d5ce3b889bf90b513df9.