What kind of an O2 system is on the superduke? A lambda sensor in the exhaust or air hoses at the engine heads?
In the lambda case: get rid of it
In case of the last option the only thing is does is adding clean air to the exhaust gasses. This way there will be a slight better burn of polluting exhaust gasses (in the exhaust itself) but most important: relatively less pollution in the gasses coming out of the exhaust, simply because there is more fresh air added to the 'dirty' gasses.
To tune the motor on the dyno this extra fresh air in the exhaust will be unwanted because it f*cks up the true A/F line, making it look like the bike runs lean on low RPM and low throttle openings whil the mixture could actually be perfect. Therefore you need to block this hose on the dyno (surgeon clamps work fine) but you can release this afterwards since this does not influence the power. Only thing it could do is a little exhaust popping when engine braking.
I wouldn't put it back on, it's ugly, unnecessary and extra weight