substitute (or sub)

deputy-an assistant with power to act when his superior is absent

foster – replacement parent

replacement-a person or thing that takes or can take the place of another

surrogate- a person appointed to represent or act on behalf of others

viceregent (substitute ruler)

ersatz (adjective meaning artificial and inferior substitute)

back-up-spare person or thing held in reserve.

reserve- something kept back or saved for future use or a special purpose

stand by-the one available or ready for a certain function or service

second string-an alternative resource or course of action in case another one fails.

temp- a worker (especially in an office) hired on a temporary basis

relief- someone who takes the place of another (as when things get dangerous or difficult)

subrogate (that’s a legal sleight of hand)- substitue one creditor for another, as in the case where an insurance company sues the person who caused an accident for the insured

alternate- someone who takes the place of another person

double (as in twin or duplicate)

“acting” as in the “acting” replacement

factice (realistic copy substituted for the original, from French)

locum tenens -physician or clergyman who subs temporarily for another physician or clergyman

understudy – actor/replacement

representative-a person who represents others

auxiliary-someone who acts as assistant functioning in a subsidiary or supporting capacity

(shall i go on?)

Please note there are definite nuances-they aren’t all exact synonyms by any means!!!

double

stunt double

body double

relief pitcher

pinch runner

wing man (?)

While not actually the type of thing you are looking for doppelgänger is an interesting word meaning a look alike or double. In the vernacular, “Doppelgänger” has come to refer (as in German) to any double or look-alike of a person—most commonly an “evil twin”. The essential meaning of the German word is “doublewalker”, someone who is walking the same way as you do. The word is also used to describe the sensation of having glimpsed oneself in peripheral vision, in a position where there is no chance that it could have been a reflection. They are generally regarded as harbingers of bad luck. In some traditions, a doppelgänger seen by a person’s friends or relatives portends illness or danger, while seeing one’s own doppelgänger is an omen of death. In Norse mythology, a vardøgr is a ghostly double who precedes a living person and is seen performing their actions in advance.

Wet nurse