Ports parser doesn't filter duplicate ports

[stufus]

Currently, adding a port specification (e.g. set PORTS 22-25,23) will add '23' twice. Internally, this is because it generates a per-port array so that the work can be divided up between multiple threads. Deduplicating the array on each addition caused a significant speed penalty. Just need to experiment with ways of deduping the array.

Shouldn't be a hard problem to fix.