[issue2756] urllib2 add_header fails with existing unredirected_header

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

Bug needs to be verified on 3.10 or 3.11.

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nosy: +terry.reedy
versions: +Python 3.10, Python 3.11 -Python 2.6, Python 2.7

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<https://bugs.python.org/issue2756>
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