[1.7.x] Fixed #23621 -- Warn for duplicate models when a module is reloaded.

Previously a RuntimeError was raised every time two models clashed
in the app registry. This prevented reloading a module in a REPL;
while it's not recommended to do so, we decided not to forbid this
use-case by turning the error into a warning.

Thanks dfunckt and Sergey Pashinin for the initial patches.

Backport of 8c4ca16c65 and b62f72498a from master