Deuteronomy 21:17

but he shall acknowledge the firstborn , the son of the unloved , by giving him a double portion of all that he has , for he is the firstfruits of his strength . The right of the firstborn is his.

He must acknowledge the son of his unloved wife as the firstborn by giving him a double share of all he has. That son is the first sign of his father`s strength. The right of the firstborn belongs to him.

But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath : for he is the beginning of his strength; the right of the firstborn is his.