Quoting Hendrik Boom (hendrik@???):

> Can I hope that it won't append a Reply-To: header if there is already one?

I really have no idea.

Mailman's versions of the last few years have, if as listadmin you
have the poor judgement to enforce Reply-To munging via the admin WebUI,
done that forcing _additively_, appending the forced header to any
existing one supplied by the sender (e.g., as a second Reply-to
addressee, comma-delimited). So, I suspect the ones added for
DMARC mitigation would do likewise.

If you wish the answer to that specific question, maybe you should ask
the GNU Mailman developers.