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Boxy, who's making it up? It's what religion teaches. If Adam and Eve were the first and only two people on earth, and parents of all humanity, then logically incest must have occurred many times. That's the only possible way the human race could have arisen.
However, although that's what religion teaches, it isn't what the bible says. The earth was populated before the appearance of Adam and Eve - so you're quite right - there were other people around. We know there were certainly people inhabiting the Land of Nod (wherever that was!), because that's where Cain found his wife. (As far as we know Abel didn't marry anyone - he was already dead).