A character is transported (or otherwise somehow travels) to a Parallel Universe, or to another dimension, where they meet version of themselves. The counterpart may look similar, or have a similar name (possibly made up). They may have similar interests or behavior, or may even be gender flipped. Usually, though, it is a combination of any of these. If the character is time traveling, expect the "counterpart" to be the traveler's relative, though this isn't mandatory. Related to Cuckoolandian Counterpart, except that oneself in another universe isn't necessarily wacky or strange, they just bear a striking resemblance to the particular character. Compare Future Me Scares Me and My Future Self and Me.
Hello, Unknown Troper. You'll need to get known to lend a hand here.