How do I get rid of double bounces?

It depends on what you mean by "ger rid of." If you just want to delete all of your double bounces, you can do this:

# echo oblivion > /var/qmail/control/doublebounceto

# echo '#' > /var/qmail/alias/.qmail-oblivion

Restart qmail, and your double bounces will be sent into oblivion. (If you have an actual user called oblivion, you'll probably want to use another name.)