"We could ask what might happen if we were to meet one of our copies. You might think this was just like shadow boxing in the mirror but there is no reason to think that any one of your doubles would act as you do. You may both have identical histories up until the moment of encounter, but confronted with a new situation you might respond differently, for the first time, just as two identical twins might do. In the future your experiences and choices would become increasingly different. Yet elsewhere in the infinite universe there would have to be a never-ending series of copies of each of us making the same future decisions and being in every respect identical. It is as if every possible decision that we could have taken at every moment is actually taken. There is always someone somewhere who lives a past life identical to my own and then takes one of all the possible decisions open to me about what to do next. They are always more numerous than those who continue to choose like I do. Others are alarmed at the ethical consequences of a universe where all possible sequences of events take place regardless of their consequences. All possible virtual histories are acted out for real. There are histories where evil always overcomes good and others where that is even thought to be a good thing."
-The Book of Universes. John D. Barrow.