Sieve duplicate detection

[Bruce Bodger]

Good day, Gents.

We are running:
dovecot 1.2.17
dovecot-1.2-sieve-0.1.19
dovecot-1.2-managesieve-0.11.13
postfix 2.4.3 
on OS X 10.5.8 server.  Only about 15 users access the server.  No problems.

My question is... what would I have to upgrade, and to what version, in order to take advantage of pigeonhole sieve's duplicate detection (vnd.dovecot.duplicate) functionality?  

Thank you very much.

Bruce