Checking to see if you received a file that you sent (in LISTSERV) to
prevent mail loops is inadequate; you may be external to the actual loop
itself.  you-->b-->c-->d or some such construct will do it as well (past
               |       |
               ---------
experience with Relay loops).  If you could put a message-ID tag (or some
similar tag) on a mailfile the *first* time it is received, it could be
made to work.  Record the x-most recent message ID's received (or that
you originated) in a pushdown list.  When you receive a file, if it has
a tag already, and the tag matches one already in the list, this file has
no doubt been here already and should be transferred to the postmaster.
You would have to allow this tag to propagate and not strip it like some
other tags (don't get me wrong, I *love* getting mail without sixteen sets
of 'received by:' headers, but it makes peer mail look curiously 'original').