People are fundamentally bad at using 'reply-all' - or perhaps it is that 'reply-all' itself is bad. Either way, reply-all to lists will reply to the list AND to the poster, who most likely is a member of the list anyway. So, s/he gets two copies. This irritates me because I have to end up removing duplicates myself, manually.

Worry not, here comes procmail and formail to save the day. The following 2 rules will put duplicates in a 'duplicates' folder.

# Keep track of message IDs.
:0Whc
| formail -D 8192 messageid.cache

# I don't want to see duplicate messages
:0a
duplicates

A special note on duplicate detection. Every email (hopefully) will have a Message-ID. Replies will hopefully have an In-Reply-To or a References header that specified the Message-ID that it is in response to. This is how MUA's (mail user agents) know how to sort mail by "thread."

At any rate, formail -D tells formail to look in the cache (messageid.cache) for an existing message id. If it's not found, it is stored and formail exits with failure status. If it is found, formail exits with a success

Read the procmailrc manpage if you want to know what the W, h, c, or a flags mean on rules.