It's a simple tale: Person photocopies self, or travels to an Alternate Universe, or another time. Person meets self. Both selves "make beautiful music" together. After all, what better person to be most intimate with? Probably more prevalent in fanfiction than in original works, but it's out there. Technically, any intimacy between a character and their Enemy Without will be this. Expect much argument amongst the characters, fans, and oftentimes even both as to whether this qualifies as actual sex, incest, or masturbation. Also known as selfcest, autoincest, or incesturbation.