Duplicate mails, since updating to 3-0-6?

I’ve just updated our RT from 3-0-2 to 3-0-6, mainly to get the CLI, and
I’ve found that several people have reported getting duplicate mails from
RT - at least the ‘thanks for your query, your ticket is ####’ mail.

Has anyone else had a problem with this?
I’m running RT with mysql and qmail on FreeBSD, if it’s relevant.