Ummm, okay, I'll be the one to say it. There are quite a few different types of twins. Maybe our geneticist, Ustwo, can help me out.
There is such a thing as fraternal twins with different fathers. Superfecundation is the term for it. Apparently, about 1 in 400 dizygotic sets of twins born to white women in the US is bipaternal.
If anyone is interested:
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