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.

Below are copies of two occurences of this from the LISTSERV logs my service
provider sent me. I have removed all subscribers' addresses for their
privacy:

14 Mar 2006 03:54:25 Processing mail from ***.COM for atsalist
14 Mar 2006 03:54:26 Distributing mail ("ATSALIST") from
owner-atsalist@ATSALIST.COM...
14 Mar 2006 03:54:26 Mail posted via SMTP to ***.COM
....ETC....
14 Mar 2006 03:54:26 Mail posted via SMTP to ***.COM
14 Mar 2006 03:54:26 Mail posted via SMTP to ***.COM
14 Mar 2006 03:54:26 Done - 1 outbound file (308 rcpts).
14 Mar 2006 03:54:26 Distributing mail ("ATSALIST") from
owner-atsalist@ATSALIST.COM...
-> Skipped: ***.BITNET
-> Skipped: ***.BITNET
14 Mar 2006 03:54:26 Mail posted via SMTP to ***.COM
14 Mar 2006 03:54:26 Mail posted via SMTP to ***.COM
...ETC...
14 Mar 2006 03:54:26 Done - 1 outbound file (433 rcpts).
14 Mar 2006 03:54:26 Message DISTRIBUTEd to 769 recipients.
14 Mar 2006 03:54:26 Sent information mail to ***.ORG
14 Mar 2006 03:54:58 Processing file 92653 from [log in to unmask]
14 Mar 2006 03:54:58 From [log in to unmask]: X-ADMMAIL
OWNER-ATSALIST
14 Mar 2006 03:54:58 Processing file 92655 from [log in to unmask]
14 Mar 2006 03:54:58 -> Removing unwanted MIME message parts...
14 Mar 2006 03:54:58 Sent information mail to ***.ORG
14 Mar 2006 03:55:09 Processing file 92657 from [log in to unmask]
14 Mar 2006 03:55:09 -> Removing unwanted MIME message parts...
14 Mar 2006 03:55:09 -> Rejected:
* Your message is being returned to you unprocessed because it appears to
have
* already  been distributed  to the  atsalist 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.  If you have  a
good
* reason to  resend this message  to the list  (for instance because  you
have
* been notified  of a hardware  failure with loss  of data), please  alter
the
* text  of the  message in  some way  and  resend it  to the  list. Note
that
* altering the "Subject:" line  or adding blank lines at the  top or bottom
of
* the message is not  sufficient; you should instead add a  sentence or two
at
* the top  explaining why  you are  resending the message,  so that  the
other
* subscribers understand why they are getting two copies of the same
message.
14 Mar 2006 03:55:09 Sent information mail to ***.COM
14 Mar 2006 03:55:21 Processing file 92659 from [log in to unmask]
14 Mar 2006 03:55:21 From [log in to unmask]: X-ADMMAIL
OWNER-ATSALIST
***.COM
14 Mar 2006 03:55:21 Processing file 92661 from [log in to unmask]
14 Mar 2006 03:55:21 Sent information mail to ***.COM
14 Mar 2006 03:56:42 Processing file 92663 from [log in to unmask]
14 Mar 2006 03:56:42 From [log in to unmask]: X-ADMMAIL
OWNER-ATSALIST
***.COM
14 Mar 2006 03:56:42 Processing file 92665 from [log in to unmask]
14 Mar 2006 03:56:42 Sent information mail to ***.COM
14 Mar 2006 03:59:50 Processing file 92667 from [log in to unmask]
14 Mar 2006 03:59:50 From [log in to unmask]: X-ADMMAIL
OWNER-ATSALIST
14 Mar 2006 03:59:50 Processing file 92669 from [log in to unmask]
14 Mar 2006 03:59:50 -> Removing unwanted MIME message parts...
14 Mar 2006 03:59:50 Sent information mail to ***.COM

************* Second Occurence *************

15 Mar 2006 06:50:38 Processing mail from ***.ORG for atsalist
15 Mar 2006 06:50:39 Distributing mail ("ATSALIST") from
owner-atsalist@ATSALIST.COM...
15 Mar 2006 06:50:39 Mail posted via SMTP to ***.COM
15 Mar 2006 06:50:39 Mail posted via SMTP to ***.COM
...ETC...
15 Mar 2006 06:50:39 Done - 1 outbound file (310 rcpts).
15 Mar 2006 06:50:39 Distributing mail ("ATSALIST") from
owner-atsalist@ATSALIST.COM...
-> Skipped: ***.BITNET
-> Skipped: ***.BITNET
15 Mar 2006 06:50:39 Mail posted via SMTP to ***.ORG.
15 Mar 2006 06:50:39 Mail posted via SMTP to ***.NET.
...ETC.....
15 Mar 2006 06:50:39 Done - 1 outbound file (437 rcpts).
15 Mar 2006 06:50:39 Message DISTRIBUTEd to 775 recipients.
15 Mar 2006 06:50:39 Sent information mail to ***.ORG
15 Mar 2006 06:51:00 Processing file 98707 from [log in to unmask]
15 Mar 2006 06:51:00 From [log in to unmask]: X-ADMMAIL
OWNER-ATSALIST
15 Mar 2006 06:51:00 Processing file 98709 from [log in to unmask]
15 Mar 2006 06:51:00 -> Removing unwanted MIME message parts...
15 Mar 2006 06:51:00 Sent information mail to ***.COM
15 Mar 2006 06:51:12 Processing file 98711 from [log in to unmask]
15 Mar 2006 06:51:12 -> Removing unwanted MIME message parts...
15 Mar 2006 06:51:12 -> Rejected:
* Your message is being returned to you unprocessed because it appears to
have
* already  been distributed  to the  atsalist 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.  If you have  a
good
* reason to  resend this message  to the list  (for instance because  you
have
* been notified  of a hardware  failure with loss  of data), please  alter
the
* text  of the  message in  some way  and  resend it  to the  list. Note
that
* altering the "Subject:" line  or adding blank lines at the  top or bottom
of
* the message is not  sufficient; you should instead add a  sentence or two
at
* the top  explaining why  you are  resending the message,  so that  the
other
* subscribers understand why they are getting two copies of the same
message.