> However, had message IDs been the RFC822 message ID, then it would have > been possible to receive new mail on each node. Since the IDs would not > collide, that would be OK. If you did get the same ID delivered to both > somehow, it should be the same mail. If not, it violates the RFC, so > then you decide to replace one of them or change one of them and move on.
Perhaps the same mail, but with different Received: headers, and possibly with mailing list software headers added. IMO, "Message-Id:" is not strong enough of a UIDL, *especially* for list owners.