[issue38843] Document argparse behaviour when custom namespace object already has the field set

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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nosy: +miss-islington
nosy_count: 4.0 -> 5.0
pull_requests: +22535
pull_request: https://github.com/python/cpython/pull/23668

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38843>
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