To what extent is a doppelganger supposed to look like you?

Are they supposed to look like an identical twin with only slight differences like freckles and birth marks? Or can they look very similar too you, but not quite like an identical twin. (ie: like the pairs brothers or sisters who look VERY similar, but aren't necessairly identical twins)To what extent is a doppelganger supposed to look like you?
Doppelg盲nger is the ghostly double of a living person, a sinister form of bilocation. So it is any living person, who looks like a spirit.

Godspeed.