I've seen, but rarely, bounced mail from a subscriber address "x" appear
to be re-posted by the originator "y."  Often times it will bounce,
including the reason that you said -- a duplicate, but sometimes because
the list headers appear in the text and that triggers LISTSERV(R)
anti-mail loop procedures.

The frequency of this error has been greatly diminished over
time.  Having a daily threshold helps too.  When I find the offending
subscriber, I delete them with a note to their postmaster about their
severely broken email system.

Bah and Humbug <just kidding>

/Pete