Given my experiences, I assume that it's a lot more common than people think. I have never seen anybody who looks exactly like me, but years ago when I was at university, I thought I saw my sister walking past me wearing brightly-coloured clothing that she would never wear. I realised that it was not her, and other people thought my sister had gone to another school that she had never attended. I have discovered that the family of a girl that I went to school with seems to have an amazing amount of coincidences relating to birth and death dates that correlate either closely or symbolically with our family's. Her daughter, a first-born like me, has a birth date very similar to mine and her first and middle names are exactly the same length as mine. Even more strangely, her husband has a birth date very similar to my father's and her parents died on similar dates to my grandparents in New Zealand. In a world of over 7 billion people I guess that there are lots of Doppelgangers, it's just that they are very spread out and it's rare to encounter them.