Twinception

Fraternal twins (one male and one female) who have fraternal twins themselves (one male and one female) who have fraternal twins themselves (one male and one female)... and so on

"Hey John, I was talking to your sister, and from your family tree, it looks like you guys had a pretty bad case of twinception, like 6 generations in-bred"
"That's no big deal, my twins might be in high school and having sex on a regular basis, and one of them have double-down syndrome, but it isn't any different than my household growing up... Like father and mother, like son and daughter i guess."

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