Sieve configuration error in Dovecot

So, I tried all of the documented variants of enabling the duplicate extension in dovecot for sieve.

But in the end I constantly get:

error: require command: unknown Sieve capability `duplicate'.

My last configurations was:

plugin { quota = dict:user::file:/var/vmail/%d/%n/.quotausage sieve=/var/vmail/%d/%n/.sieve sieve_extensions = +vnd.dovecot.duplicate sieve_max_redirects = 6 } 

EDIT: This is also a little private bug tracker and how to for sieve duplicates.