[Dovecot] Deliver, sieve and duplicate message

[jean-Noël Chardron]

Hello,

Some users subscribe to several mailing lists which causes them to have 
the same message several times what they do not want .
Today we use procmail as delivery agent to discard duplicate message, we 
want to migrate to dovecot deliver + sieve
I read a previous post about the duplicate messages : 
http://www.dovecot.org/list/dovecot/2007-March/021085.html
But I do not understand if this feature exists and how to activate
Some explanations or documentation would be welcome.

thank you,

-- 
Jean-Noel Chardron