"Doubles for" is an expression used to indicate that someone or something is standing in for another person or thing. There are multiple synonyms for this phrase, including substitute, replacement, stand-in, proxy, and surrogate. Substitute suggests someone or something that fills in for someone else, while replacement is often used to indicate someone who takes the place of someone who is absent or no longer able to perform their duties. Stand-in and proxy are similar in that they suggest someone who is acting as a representative or proxy for another person or organization. Finally, a surrogate is typically used to indicate someone or something that is being used as a substitute for another.