The current blacklist-helper script adds a rule using 'ipfw add -q ...'
when a ipaddress/port needs to be blocked. The 'ipfw' command,
when using '-q', it not only suppresses output messages, changes the
behaviour so that it allows duplicate rules!

So, probe for an existing rule first, and if that probe fails, only then add
the rule.

Tested in a virtual machine, seems to work for me, but I am not an ipfw expert.