[issue14856] argparse: creating an already defined subparsers does not raises an exception

[Braden Groom]

Change by Braden Groom <braden.groom at gmail.com>:


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pull_requests: +9481, 9482, 9483
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue14856>
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