There's also the ST:TNG episode "Second Chances". http://en.wikipedia.org/wiki/Second_Chances_%28Star_Trek:_The_Next_Generation%29 http://en.wikipedia.org/wiki/

There's also the ST:TNG episode "Second Chances". http://en.wikipedia.org/wiki/Second_Chances_%28Star_Trek:_The_Next_Generation%29 http://en.wikipedia.org/wiki/Second_Chances_%28Star_Trek:_Th... where Riker is duplicated. (There's also the good/bad Kirk in 'The Enemy Within', and the whole mirror universe concept, but those aren't duplicates.)