[Users] Dealing with message duplication on mailing lists

[mlist]

Hi,

I am experiencing something quite annoying on other
mailing lists:

There is this pattern of people (obviously) hitting
"reply to all" instead of replying to the mailing list
address only. The result is: in threads started by me
I receive all messages twice (once from the list and
once directly from the sender) with only a few not
duplicated (those sent correctly by people who know
how to send properly).

How do you guys deal with that? Is there a way to
filter out and clean up duplicates?