A look-alike is someone who looks almost exactly like another person. Running into your look-alike in a big city would be quite shocking.
Some look-alikes are related, like identical twins or cousins who resemble each other to an uncanny degree. Other look-alikes are so similar to celebrities that they can make a living impersonating them. If you are a Michael Jackson look-alike, you could dress like the singer and perform at a talent show, lip syncing his song "Billie Jean" and attempting the moonwalk. Look-alike is an American word, coined around 1937.