Deuteronomy 21:15-17

When a man has two wives, one loved and the other hated, and they both give him sons, but the firstborn is from the hated wife, at the time he divides the inheritance with his sons he must not treat the son of the loved wife as the firstborn, cutting out the son of the hated wife, who is the actual firstborn. No, he must acknowledge the inheritance rights of the real firstborn, the son of the hated wife, by giving him a double share of the inheritance: that son is the first proof of his virility; the rights of the firstborn belong to him. * * *