Here is another way to remember that the filter list has already been initialized. I made a new immortal _warnings.filters_initialized flag at the C level. It is actually a list so that it can be mutated and remembered across module reloads, but it is either empty (evaluates as false), or a single element: [True].

Also, Python 2 does get duplicated filters, but I guess there is not test that exposes it.

$ python2 -Wall
. . .
>>> import warnings
>>> len(warnings.filters)
5
>>> reload(warnings)
<module 'warnings' from '/usr/lib/python2.7/warnings.pyc'>
>>> len(warnings.filters)
6

I agree there is no need to change Python 2 at this stage.