Deuteronomy 21:16-17

16then on the day when ahe assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the firstborn in preference to the son of the unloved, who is the firstborn, 17but he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is bthe firstfruits of his strength. cThe right of the firstborn is his.

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