Alison also posted up a blogg on doppelganger. These are apparently meant to be ones exact double. They too could be classed as uncanny. No one knows if they are real or if they are ghosts, or even oneself in the future.
I've personally never seen anyone who looks like me, but I do have a twin. A friend of mine who was two years below me at secondary school was said to look like me. Although she was two years younger, slimmer, slightly shorter and was the total opposite to me (look wise) no one would take the answer 'we're not related at all'. They always insisted we were at least cousins because they thought we look like each other. We were even mistaken for twins more times than I could count. Not quite a doppelganger, but she was my 'evil twin'. It was because of her I eventually gained the nickname Lev.