After we learn that inheritance to sons is to be emphasized, we perforce must follow the first approach, viz.: Since he inherits with one (son) and he inherits with five, then just as when he inherits with one, he receives a double portion of one, so, when he inherits with five, he receives a double portion of one. And it is written (I Chronicles 5:1) "And the sons of Reuven, the first-born of Israel (i.e., Jacob).
For he was the bechor, but when he defiled his father's bed, his birthright was given to the sons of Joseph." And it is written (Ibid. 2) "For Judah prevailed over his brothers (to inherit the kingship) and the ruler was to come from him, and the bechorah (the status of first-born) was given to Joseph." And it is written (Bereshith 48:22) "And I (Jacob) have given to you (Joseph) an additional portion over your brothers."