In fact, the situation only gets tricky when you're dealing with peered lists. If you have a unique list, it will take the first posting and reject any subsequent duplicate. Of course the sender might wonder why messages are duplicated, and he's not the only one wondering :-), but the other subscribers will never know about the problem. Now let's say you have a peered lists with 3 hosts, A--B--C. Murphy's law being what it is, the duplication will not occur between the poster and A, but between A and B. From the point of view of A, the situation is perfectly normal, it got one copy and distributed it, then never heard about it again. B on the other hand got 20 copies from A, so it has processed one and returned 19 to the sender, who now wonders why LISTSERV@B is complaining about something which he (the sender) had posted to LISTSERV@A in the first place - obviously, the software must be broken in some way :-) That was the "simple" case where all the peers involved are running with 16E-003o. Now let's assume B is running the old logic: it gets 20 copies, it distributes 20. A never gets them back since they were all coming from A; C does get 20 copies. Case 1: C doesn't run 16E-003o. Well the sender will never know there was a problem, nor will the list owner. Half of the subscribers will start complaining about getting 20 copies of everything, the other half will wonder why nobody has called the asylum yet, the list owner will be at a loss as to which part of the user population he should believe and what, if any, he should do about it (the answer is: get B and C to run 16E-003o :-) ). Case 2: C runs 16E-003o. We have basically the same situation whereby some of the recipients get 20 copies and some don't, except the sender will find out about the problem. After a week or so the users will have learnt all the gory details and the B recipients will start demanding to be moved to A or C so that they can stop getting all that junk mail. In other words, the users will demand that peer B be upgraded or removed; if peer B is running 1.5o, the situation gets more tricky as it is unlikely that it will be upgraded within a reasonable amount of time. The list owner will then have to decide whether or not he wants to keep the peer, based on a real-world problem which is, admittedly, not the fault of the staff at B and for which they there is easy solution, even with the best goodwill in the world. Eric