This is TYPICAL of a subscriber whose mail system is redirecting ("resent") a distribution back to the list.  Thus, to the LISTSERV processor it appears to be a dup and bounces it back to the very original poster.

I.e., this is a SMTP related problem and nothing to do with LISTSERV (other than it prevented a mail loop).

You need to carefully inspect the bounced mail headers (you must have full headers generated) and look for signs of the host that is redirecting it (the distritubed posting) back to the list and then NUKE that subscriber, with NO QUESTIONS ASKED.  

Then clue-x-4 the mail admins (if it isn't a customer config error) indicating that their mailer is FUBAR and that you will not allow anyone from that host to subscribe to any of your lists until they fix it.

/Pete Weiss
been there, done that

 At 17:12 4/12/2006 Wednesday, Michelle Wilbers wrote:
>When any of my list subscribers send a post to the list, LISTSERV sends back
>two messages. One says the message was successfully posted (and it appears
>in the archives). The second message, which comes simultaneously with the
>first, says the post was not accepted, but was deleted because it was a
>duplicate.