compeer, equal, peer, match - a person who is of equal standing with another in a group

locum, locum tenens - someone (physician or clergyman) who substitutes temporarily for another member of the same profession

stunt man, stunt woman, double - a stand-in for movie stars to perform dangerous stunts; "his first job in Hollywood was as a double for Clark Gable"

alternate, surrogate, replacement - someone who takes the place of another person