PyOS_AfterFork() is used by one code, but the "before" and "parent" handlers are registered by the other code (likely the code of other library). The author of the program that uses both libraries (the one that uses PyOS_AfterFork() and the one that registers handlers) can notice the warning and report the bug in the first library. I think silently skipping registered handlers would be worse.

Let allow the user to control the behavior by setting the warnings filter. If the corresponding warnings are ignored, nothing happen, if they are errors, the child is exited, otherwise the warning message is output on stderr as for other warnings.