User: Handle DuplicateHostmask exception in hostmask.add.
User: Handle DuplicateHostmask exception in hostmask.add.

Signed-off-by: James Vega <91ae71a1168cea0b65ac9975da69d5a7753116be@users.sourceforge.net>

haxwithaxe/supybot,ProgVal/Limnoria-test,Ban3/Limnoria,Ban3/Limnoria,buildbot/supybot,frumiousbandersnatch/supybot-code,ProgVal/Limnoria-test,jeffmahoney/supybot,raboof/supybot