To stop getting double bounce messages-
[root@server /]# config setprop qmail DoubleBounceTo devnull
[root@server /]# signal-event email-update
If you really geeky

You can check the database if you like...
[root@server /]# config show qmail
qmail=service
DoubleBounceTo=devnull
MaxMessageSize=15000000
status=enabled
Expand only the doublebounceto template....
[root@server /]# expand-template /var/qmail/control/doublebounceto
Check /var/qmail/control/doublebounceto is set to devnull.....
[root@server /]# cat /var/qmail/control/doublebounceto
devnull
And restart qmail 'manually'....
[root@server /]# svc -t /service/qmail