Simulacrums

The mentioned differences are usually swapped primary hand (e.g. left-handed instead of right), slight discolorations, such as darker hair or eyes, or it can be something more noticeable such as an ominous aura/halo or some features not present in the "original".

Mirror image refers to a somewhat common instance in fiction where one fights an almost literal mirror image of themselves, any wounds being "reflected" upon themselves and such. Commonly the only solution to beat such is to sheathe your weapon and pass without aggression, though there's room for alternate case where you must defeat the simulacrum with violence despite the indication that it would be fatal to you too (essentially overcoming a perceived fallacy that you're fighting yourself rather than someone else, inverting the initially described case).

Unlike doppelgangers, simulacrums are unable to change their shape (by default, usually only able to do so if the protagonist can too) and their appearance is not maintained for the purpose of misleading anyone, rather it is "natural" state for them.

See also: doppelgängers, alternate self