cc modification due to nodup setting breaks DKIM & thus DMARC
I actually ran into this in mailman 2, but looking at the mailman 3 code, it seems to be there too. See https://bugs.launchpad.net/mailman/+bug/1845751
It looks like a logically equivalent patch should be created for src/mailman/handlers/avoid_duplicates.py . I don't run mailman3 yet and I don't want to start just to write the patch.