[Dovecot] Duplicate suppression from remote lists?

Hi, I have a setup where users on my server who are subscribed to several lists on a (remote) weirdly-configured listserver are getting many duplicate e-mails. I know why this is happening; but I don't have control over that listserver so I can't fix it. Is there a way for me to suppress duplicates from within Dovecot? The duplicates do all have the same (otherwise-unique) message ID.

"dovecot --version" gives 1.0.rc15.  I'm using exim for SMTP.  I'm also 

using dovecot-lda, with the Sieve plugin. There's a rather old thread on this list that mentions that dovecot-lda+Sieve should be able to suppress duplicates; it's not doing so automagically, though...

Thanks, Adam