Duplicated Co-authored-by on backport PR merged manually

[vstinner]

When the bot creates a backport PR, the "Co-authored-by" line is duplicated. Example with the backport PR python/cpython#26290.

I merged this PR manually (see bug #458), I didn't touch the commit message. I expect that the commit message would be the same if the PR would be merged automatically, but I'm not sure about that.

Merged commit with duplicated Co-authored-by:

commit 50b0d148a68072292832eb69bdf1815b8059355f (HEAD -> 3.10, upstream/3.10)
Author: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date:   Fri May 21 15:50:11 2021 -0700

    bpo-44184: Fix subtype_dealloc() for freed type (GH-26274) (GH-26290)
    
    Fix a crash at Python exit when a deallocator function removes the
    last strong reference to a heap type.
    
    Don't read type memory after calling basedealloc() since
    basedealloc() can deallocate the type and free its memory.
    
    _PyMem_IsPtrFreed() argument is now constant.
    (cherry picked from commit 615069eb08494d089bf24e43547fbc482ed699b8)
    
    Co-authored-by: Victor Stinner <vstinner@python.org>
    
    Co-authored-by: Victor Stinner <vstinner@python.org>

The PR description was:

Fix a crash at Python exit when a deallocator function removes the
last strong reference to a heap type.

Don't read type memory after calling basedealloc() since
basedealloc() can deallocate the type and free its memory.

_PyMem_IsPtrFreed() argument is now constant.
(cherry picked from commit 615069eb08494d089bf24e43547fbc482ed699b8)


Co-authored-by: Victor Stinner <vstinner@python.org>

<!-- issue-number: [bpo-44184](https://bugs.python.org/issue44184) -->
https://bugs.python.org/issue44184
<!-- /issue-number -->

Commit message of the commit created by the bot:

 bpo-44184: Fix subtype_dealloc() for freed type (GH-26274)

Fix a crash at Python exit when a deallocator function removes the
last strong reference to a heap type.

Don't read type memory after calling basedealloc() since
basedealloc() can deallocate the type and free its memory.

_PyMem_IsPtrFreed() argument is now constant.
(cherry picked from commit 615069e)

Co-authored-by: Victor Stinner <vstinner@python.org>

[Mariatta]

It's not miss-islington that added the second co-authored-by.
It was perhaps added by GitHub when you did the manual merge?

miss-islington added the co-authored-by only when it created the PR, but not during merge time.
Looking at this commit (python/cpython@4a549e4), it looks like miss-islington correctly added only one co-authored-by trailer.

I'm closing since it doesn't seem to be a problem with miss-islington.