There's a rather simple recipe in the procmail examples for filtering out duplicate e-mail messages using their message ID:
:0 Whc: msgid.lock
| formail -D 8192 msgid.cache
I'm wondering, and Google fails me, is there an easy way to achieve this same effect* in an exim filter?
* Or something close. Specifically the ability to remember what message IDs have already been seen, and if the message ID has been seen already, deliver the duplicate message somewhere else.