Fix duplicate values in Config.known_args_namespace for append actions

[EternalRights]

Ran into #13484 while looking at the config parsing code. The third parse_known_args call passes self.known_args_namespace as the namespace, which already has values from the second parse. For action="append" options like -W, this causes duplicates.

Fix is to use copy.copy(self.option) instead, same as the first two calls. This way each parse starts from a clean namespace and append actions don't accumulate across multiple parses.

Not sure if there was a reason the third call used known_args_namespace directly, but from what I can tell the only new things after the second parse are plugin-registered options, and those would still be picked up correctly with a fresh namespace.

Closes #13484