To trigger a nuclear apocalypse, the guy doesn't need to trigger all the nukes himself. Triggering a few to exploit existing fault lines is enough to watch everyone destroy themselves. For example - India Pakistan, India China, US China, US North Korea, US Russia, Israel Iran (rumored to be working on nukes).
At the rate of 5 per minute, duplicator can produce 288 clones per day. However, things like credit card, identity card etc tie into an external database (of banks, govt), so they won't work. So, your guy needs to go old school, with cash etc :P.
Since he can have the right knowledge, he can be someone who already knows how to compromise a nuke carrying submarine with the right number of clones. This is possible if he knows schedule etc of the sub's stops, and knows enough to operate and flee away with it, and launch the nukes.
This way, he is armed, and mobile.
Once done, all that is needed is to trigger nuclear strikes on individual cities, and watch the world destroy itself.
Of course, the best place for a duplicator in such a case will be some hidden location within the sub itself.
However, if you want the guy to physically grab all the nukes and trigger them, that would be a difficult task - there are supposedly 8 nuclear armed countries, with over 16000+ warheads as per wikipedia. even if these are stored 10 per location, you would have 1600 really strongly defended locations, busting which will be an impossible feat given the constraints involved (intelligence, logistics, coordination, etc)