Charles Ellicott 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)

Deuteronomy 21:15–17. THE BIRTHRIGHT.

One beloved, and another hated —that is, one preferred above the other, according to the idiomatic use of this phrase in Hebrew.

A double portion. —Literally, the mouth of two, that is, two shares. Supposing there were four sons, the estate would be divided into five shares, and the firstborn would take two. So Jacob said to Joseph (Genesis 48:22): I have given you one portion above your brethren. The birthright of which Reuben was deprived for ill conduct, was given to Joseph’s sons (1 Chronicles 5:1). So Elisha said to Elijah before they were parted. I pray you let a double portion (the first-born’s share) of your spirit be upon me (2 Kings 2:9).