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A simple reading of the record of Genesis 3:1,2 would not lead one to think that they were twins.
Genesis 3:1,2 [NIV] Adam lay with his wife Eve, and she became pregnant and gave birth to Cain. She said, "With the help of the LORD I have brought forth a man." Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil.
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