Duplicate suppression, other processing at split time

[David Z Maze]

If I wind up with the same message delivered to me twice, can I cause
the second message to get dropped at split time?  (In particular using
nnimap-split-fancy, but I could probably adapt other techniques people
have for doing this.)  Is other processing possible?

The particular problem I have is people sending messages without
Message-Id to multiple lists that I'm on.  Then Gnus sorts each of the
copies into each of the lists in the To: line.  But, since there's no
Message-Id, duplicate suppression never trips, and I wind up seeing
n^2 copies of the message; if n is 3 or 4 this gets really annoying.
Another part of our local mail pipeline inserts a string that has the
right properties to be the Message-Id, so first I'd like to copy that
header value to Message-Id if there isn't already one, then do dup
suppression by Message-Id, *then* split.

(Procmail scares me -- I understand what Gnus does, and
nn*-split-fancy is kind of elegant in its own way -- but I think it is
an option, if this is something easy to do there.)

Thanks,

  --dzm