The same way
all grandchildren come into being – people’s kids have their own kids?
But no, to the best of my knowledge, there’s no doctrine or dogma on that issue. Some theologians have tried to wrap their heads around it. The notion of “not incest, per se, by definition” is one of those attempts. As I understand it, the notion isn’t that the “prohibition against incest” was lifted, but that incest is forbidden because it creates humans with certain genetic issues… and, as the idea goes, since they were “more physically perfect” than us, there wasn’t the chance of physical issues… and therefore, no incest. (Can’t say I buy that explanation, though. Still sounds kinda incest-y to me…)
Another way of explaining it is that the Genesis account isn’t trying to give a literal historical account, and therefore, there must’ve been more people – or at least, more children of Eve.
Nevertheless, if that were the case, then we’d all have DNA that proves that we arose from a single woman at the very beginning… and that’s not what science shows us.