[Exim] Exim filters and duplicate Message-IDs

    So, I've been playing with Exim filters as an alternative to

procmail, and they are indeed much nicer.  However, I can't find a way
to do the equivalent of formail -D to eliminate duplicate messages.
I've searched Google, the exim-users archive, and all the Exim docs, to
no avail.
    Well, here's the solution I came up with.  Put this before the
userforward router in the exim4 config:

<snip>
#
# dupclean router
#
# This router checks for a user's $home/.eximidcache file. This is a
# file for lsearch lookups, where the key is a message-id and the value
# is anything nonzero. If the key is found, the message is considered
# a duplicate and discarded.
#

dupclean:
  debug_print = "R: Checking message-id $header_message-id: for $local_part@$domain"
  driver = redirect
  check_local_user
  no_verify
  no_expn
  allow_filter
  require_files = $home/.eximidcache
  condition = ${lookup{$header_message-id:}lsearch{$home/.eximidcache}}
  data = # Exim filter \n\
         seen finish # Black hole
  no_more

</snip>

    The user is then responsible for populating $home/.eximidcache
with "<message-id>: 1" key:value pairs for the lsearch.  If the user
doesn't have a .eximidcache, this router isn't even run.  But folks like
me can do the following in .forward:

<snip>
logfile $home/.eximidcache
logwrite "$header_message-id:: 1"
</snip>

    This saves off the "<message-id>: 1" pair in .eximidcache.  This
can, indeed, be changed to a DBM later if one likes.  The next time the
message-id is seen by exim, the router matches and the duplicate message is
discarded.  Just like formail -D.
    Did I miss a more germane solution to this problem?  Please let
me know.

Joel

--

"Ninety feet between bases is perhaps as close as man has ever come
 to perfection."
    - Red Smith

            http://www.jlbec.org/
            jlbec@???