I sent the following to this list a few days ago, and have had a few
requests:

> If it helps, I've got a procmail filter that weeds out duplicate message
> ID's, and I've only seen one of everything.  This is different to the
> last mailling loop we had when I saw many of everything...

>From `` man 5 procmailex '':

     If you are subscribed to several mailinglists and people cross-post
     to some of them, you usually receive several duplicate mails (one
     from every list).  The following simple recipe eliminates duplicate
     mails.  It tells formail to keep an 8KB cache file in which it will
     store the Message-IDs of the most recent mails you received.  Since
     Message-IDs are guaranteed to be unique for every new mail, they
     are ideally suited to weed out duplicate mails.  Simply put the
     following recipe at the top of your rcfile, and no duplicate mail
     will get past it.

          :0 Wh: msgid.lock
          | formail -D 8192 msgid.cache


I actually use a 1MB cache, since some mail around here had a habit
of reappearing after a week or two, and the 8KB cache just wasn't big
enough...

Simon.