Filtering out duplicate messages

Hello!

I've just switched to maildrop from mailagent
(https://github.com/rmanfredi/mailagent) as mailagent has stopped
working with the latest Perl in Debian (5.40).

One feature that mailagent offered was a UNIQUE filter: if a message
is addressed to both me and a mailing list I am subscribed to, I get
two copies of the message.  The UNIQUE filter was able to process a
duplicate copy of a message in a different way from the first copy to
arrive.

Is there anything similar available in maildrop, or has someone
written some code to achieve this effect?

Best wishes,

   Julian