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Describing two characters that look, are dressed, stand mirrorically identical to each other.
Despite the name, the tag doesn't refer family relation of characters, but their mere visual connection in terms of look, clothes or pose which is either same or similar just mirrorically inverted.
If characters are actual twins add also appropriate tag. Same if both characters are actually dual persona of same character.
If picture simply contains just one character twice in mirrorically identical pose, then use symmetry tag.