Formal:
* Doppelganger: This is the classic term for someone who looks remarkably like another person. It implies a mystical or uncanny resemblance.
* Look-alike: This is a simple and straightforward term that describes someone who visually resembles another.
Informal:
* Twin: This is often used for people who look very similar, even if they aren't actually related.
* Double: This term is commonly used in movies or fiction to describe someone who is pretending to be another person.
* Mirror image: This suggests a perfect, symmetrical resemblance.
Other possibilities:
* Resemblance: This is a general term that can be used to describe any kind of similarity.
* Counterpart: This term suggests that the two people are similar in some way, but it doesn't necessarily imply a physical resemblance.
The best word to use will depend on the specific situation and the level of formality you're aiming for.