Genetically, each human being is more than 99% the same as any other human being. That is not true for all species. Two hamsters from the same mother, for example, though they may look alike to us, are more different genetically from each other than any two humans. A person from Ireland, for example, and a person from Japan are more alike than any two hamsters, even two hamsters from the same mother. The reason that a person from Japan and a person Ireland appear so different to us is due to perception--we are overly conscious of small differences. There are 10 billion different combinations of human genes. There are 7 billion people in the world (and 24 billion people who have ever lived). Genetically, the chance of an actual doppelganger occurring is a realistic possibility. Unfortunately, it doesn't seem like humans get along much better than hamsters.