Doppelgänger literally means "Double Walker" and legend says that if you were to meet or see your doppelganger, you would die soon or something really unlucky would happen to you. I think it's a very interesting concept, these are other two movies that also deal with the subject.
How similiar are the previous mentioned movies to Flowers in the Attic, you will be surprised. I will write more in depth about it in the future, as it really takes more detail since the comparison is not very obvious, actually for the most part is very cleverly hidden.