If you run unix and are unfamiliar with procmail, it's at
<ftp://ftp.informatik.rwth-aachen.de/pub/packages/procmail/procmail.tar.gz>.
# Portmaster-{users,radius,announce} (sent to the same box).
:0
* ^((Resent-)?To|Cc):.*portmaster-(users|radius|announce)(-(digest|list))?@
{
# Weed out duplicates. Use formail to see if the ID is in the
# cache; if it is, formail exits with 0. If not, formail
# adds the ID to the cache and exits with nonzero. An exit
# value of 0 means the message is considered delivered, which in
# this case means it's a duplicate.
:0 Wh: /tmp/pm-radius.lock
| formail -q- -D 8100 Mail/pm-radius.ids
# Part two: If the previous rule failed (i.e. the message isn't
# a duplicate), write it to the mailbox.
:0: /tmp/pm-radius
Mail/pm-radius
}