bpo-39716: Raise on conflicting subparser names. (GH-18605)

Raise an ArgumentError when the same subparser name is added twice to an
ArgumentParser.  This is consistent with the (default) behavior when the
same option string is added twice to an ArgumentParser.

(Support for `conflict_handler="resolve"` could be considered as a
followup feature, although real use cases seem even rarer than
"resolve"ing option-strings.)

Automerge-Triggered-By: GH:rhettinger