urllib.request.add_header fails with existing unredirected_header #47005

assignee = 'https://github.com/orsenthil'
closed_at = None
created_at = <Date 2008-05-04.15:11:37.434>
labels = ['easy', 'type-bug', 'library', '3.10', '3.11']
title = 'urllib.request.add_header fails with existing unredirected_header'
updated_at = <Date 2021-12-28.19:07:57.653>
user = 'https://bugs.python.org/zathras'

activity = <Date 2021-12-28.19:07:57.653>
actor = 'carlosdamazio'
assignee = 'orsenthil'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2008-05-04.15:11:37.434>
creator = 'zathras'
dependencies = []
files = ['10280', '11126', '29705', '29706', '29707', '50523', '50524']
hgrepos = []
issue_num = 2756
keywords = ['patch', 'easy']
message_count = 16.0
messages = ['66213', '66263', '66264', '66309', '69212', '71111', '71112', '71227', '75207', '132246', '186188', '186189', '186191', '409156', '409266', '409267']
nosy_count = 8.0
nosy_names = ['terry.reedy', 'facundobatista', 'jjlee', 'orsenthil', 'zathras', 'BitTorment', 'volodyaa', 'carlosdamazio']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2756'
versions = ['Python 3.10', 'Python 3.11']