Deuteronomy 21:15-17

15“If a man has two wives, one loved and the other neglected, and both the loved and the neglected bear him sons, and if the neglected wife has the firstborn son,

16when that man gives what he has to his sons as an inheritance, he is not to show favoritism to the son of the loved wife as his firstborn over the firstborn of the neglected wife.

17He must acknowledge the firstborn, the son of the neglected wife, by giving him two shares of his estate, for he is the firstfruits of his virility; he has the rights of the firstborn.