Lookalike

A look-alike is a living person who closely resembles another living person. A lookalike can somtimes be a doppelgänger or an evil twin. ("Thaddeus and Thor", "Cousin Bartie")

A lookalike can be somtimes be related, and also can not be at all. ("Thaddeus and Thor", "Cousin Bartie")

Often, the lookalike may have a have completely differet personality, even coming to a point for causing trouble. ("Cousin Bartie")

The Prince and the Pauper is one many stories in popular culture featuring a lookalike.