See the Reply-To Munging Harmful FAQ for further discussion. See the Reply-To Munging Harmful Forum . Reply-To without the ambiguity. This would prevent non-subscribers from receiving copies of messages. It would fragment cross-posted discussions, sending some followups to your list alone while other followups are sent to both lists.
Duplicate suppression is half of a solution to follow-up duplication.
It certainly takes away some of the annoyance. Unfortunately, it still leaves the extra address in the To line, causing many of the serious problems described above. Please let me know about any new implementations.