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it'd be selfcest
since you and your other self would be 100% identical by blood you could say it's just the purest form of incest
main difference from masturbation is that since there are two of you in the same point in time acting independently of each other, you and your other self have no control over what both selves are doing at any given point in time
compared to, say, having sex with yourself using portals or something like that