Only in Dt. The first-born of the first wife is to inherit a double portion.
Deuteronomy 21:15. two wives: Genesis 29:30 and 1 Samuel 1:6. hated: better, less loved.
Deuteronomy 21:17. acknowledge, etc.: read (with Graetz, Steuernagel, and also, though hesitatingly, Driver and Bertholet) accord the first-born, first-born rights. double portion: among the Babylonians, sons inherited equally, though the father might in his life make a special present to any son (see CH, § 165f.).
15 If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated:
16 Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:
17 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.