Deuteronomy 21:15-17

15“If a man has two wives, one loved and the other unloved, and both the loved and the unloved have born him sons, and the firstborn son belongs to the unloved wife,

16then on the day when he wills his possessions to his sons, he cannot treat the son of his loved wife as firstborn in place of the son of the unloved wife—the [actual] firstborn.

17Instead he shall acknowledge the son of the unloved as the firstborn, by giving him a double portion of all that he has, for he was the beginning of his strength (generative power); to him belongs the right of the firstborn.