> Is it possible that some server in the chain from him to the listserv
 > host is sending duplicates?  The most common cause of that is a
 > system that times out after sending the data, but before it sends
 > a 250 Message Accepted.  The sending system isn't *sure* that the
 > receiver got the message, so it tries again.

I'd be inclined to argue that today the most common cause is a mail
server somewhere which discards the envelope while doing spam or
virus checking, then reinjects the message using the header "To:".
As the "To:" header contains the list address, this results in a
"repost" warning, or duplicates.

Dennis Boone
H-Net