I subscribe to a few mailing lists which cover the same domain, but
each list's delivery schedule is quite different; some deliver
immediately, others various lag times.
More often than not, I get N copies of the same mail as the poster
will send to one and CC to the others.
What I'm looking for is a way to only get unique mails, if it comes
addressed to list1, save it. If it comes addressed to list2, save it
unless it also is addressed to list1. If it comes addressed to list3,
save it unless it also is addressed to list2 or list1.
Is there any non brute-force way to do this?