Both were the sons of "Eve", but they had different fathers. Read the tutorials.
Also, Genesis says that Cain went to the land of Nod east of Eden. It implies however that he didn't stay there. The city of Cain (which is not specifically called Eridu in Genesis) was likely not in Nod, but its actual location is unknown. Due to the prominence of Cain's line in Egypt, I'm guessing the city in question was built there.
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