Erwin Weiblen wrote, in part:
>>What can be done to find the bouncer and exclude it from the mailing
>>list?
>
> Apparently since the list has moved to another server, a lot but
> not all of the list members get the following error message each
> time they send a mail to the list:
>
> :  Subject: Rejected posting to [log in to unmask]
> :  Your message  is being returned to you unprocessed because it
> :  appears to have already been distributed to the HYPNOS  list.
> (...)
>
> Despite this error message, each mail is distributed correctly and
> from time to time there even is a mail distributed twice. The main
> problem are the irritating error messages.

These can be easy or very tough to find!  You'll need the help from your
Listserv site owner for these suggestions:

After the next post, monitor the Listserv log for the bounce coming in.
You'll recognize it as Listserv will "read it" and then send
informational mail to the poster (the message you have shown us). Now
backtrack to your mailer log (Sendmail, LSMTP or whatever) and find the
bounce mail. The mailer log should have a source system .. so now you
know where it immediately came from.  Maybe that's enough.

Alternatively, you might try a post to the list and, immediately after
distribution, "hold" the list. After the expected period of time for the
bounce to be produced, look in the Listserv spool for the awaiting
"job".  "Received:" or other header lines might give you the clue you need.

Finally, you might, after noting their settings, alter some
subscriptions to FULL822 and note the effect on the problem. As this
changes what your subscribers receive, it's a little further into the
"desperate category" of actions, but useful in this case because I think
someone or some gateway somewhere is seeing to "To:" line in a
distribution with the list address in it and, trying to be helpful, is
reflecting a copy back to the list.

Good luck! (Please let us know the disposition of the problem)
cheers, wayne