Albert Barnes Commentary Deuteronomy 21:15-17

"If a man have two wives, the one beloved, and the other hated, and they have borne him children, both the beloved and the hated; and if the first-born son be hers that was hated; then it shall be, in the day that he causeth his sons to inherit that which he hath, that he may not make the son of the beloved the first-born before the son of the hated, who is the first-born: but he shall acknowledge the first-born, the son of the hated, by giving him a double portion of all that he hath; for he is the beginning of his strength; the right of the first-born is his." — Deuteronomy 21:15-17 (ASV)

Moses did not originate the rights of primogeniture , but recognized them, since he found them pre-existing in the general social system of the East. Paternal authority could set aside these rights on just grounds (Genesis 27:33), but it is forbidden here to do so from mere partiality.