I couldn't defeat my evil twin. I am a 'only in self defense' kind of guy, so before I even knew it, it would be too late.
In fact, I would be so shit scared of him that I would not ever form any kind of relationship of any kind out of fear of what I knew he would do. I would slowly go insane until I eventually succumb to his manipulations and join him. When that happens I have only one piece of advice, RUN!