You all are thinking too much about strengths, and not enough about weaknesses--he'll (or she'll, for you females) have those, too. Sure, he'll know about "mine", but that also means he'll be equally inept at dealing with them; for instance, if he were totally committed to destroying me, that wouldn't a realistic action for my doppelganger at all, because I'm almost never totally committed to *anything*. I have to feel like I'm doing something important and useful, and my evil twin--if he were truly dedicated to destroying me--would also feel useless at doing something other than meaningful work, and "destroying alter ego" is not one in any conceivable means that I can envision. I generally hate rocking the boat, which my evil twin would also hate, and that's no way to go about destroying yourself. And on, and on...
In other words, my evil twin would be an indecisive, stubborn, overcautious, nervous wreck whose native intelligence and creativity would be neatly counterbalanced. Much more fearsome would be an evil counterpart whose strengths were *exactly* opposite to my weaknesses, in which case he wouldn't be a twin at all, but some crazed entity that has most of my marbles.