Hello!
A twin is one of two children (or animals) that are born at the same time, and have the same mother.
A doppelganger is different. It is a person who looks just like another person, a lookalike. It's an apparition of another person.
These are the differences between the two.
A twin can also look like you, just like a doppelganger, but the doppelganger won't have the same mother and they most likely won't be born at the same time.