One or probably several users have had a problem with my list server somehow
generating duplicate postings.

Are there any ideas as to where I should look for a solution to the problem?
Below, the problem is described by a user:

"
I'm having a problem with the list: whenever I send a reply to the list I
get a message stating that

"Your message  is being returned to  you unprocessed because it  appears
to have already been distributed to the  ATMOSPHERICDISPERSION list. That
is, a message with identical text (but possibly with  different mail
headers) has been posted to the  list recently, either by you or by someone
else."

But I only sent the messages ONCE, and it was posted.
"
Helge Olesen,
Nat. Environmental Research Institute, Denmark