Do not add duplicate FDs to list in duplicate_for_child()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-06.05:50:31.148>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'Do not add duplicate FDs to list in duplicate_for_child()'
updated_at = <Date 2021-02-13.05:57:18.897>
user = 'https://github.com/imaginary-person'

bugs.python.org fields:

activity = <Date 2021-02-13.05:57:18.897>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-06.05:50:31.148>
creator = 'sanchit'
dependencies = []
files = []
hgrepos = []
issue_num = 43142
keywords = ['patch']
message_count = 4.0
messages = ['386546', '386881', '386897', '386913']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'pitrou', 'davin', 'sanchit']
pr_nums = ['24461']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43142'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']