The Talmud explains that this halakha is repeated in order to include an enhancement that occurs by itself after the death of the father or husband, for instance in a case when fodder [ḥafura], i.e., produce that has grown stalks but is not yet ripe, becomes full ears, or when date flowers [shalpufei] become fully developed dates. Despite the fact that in such cases the property became enhanced by itself, without any effort on the part of the other brothers, the firstborn may not claim his double portion from that enhancement, and the same applies in the other cases mentioned in the Mishnah.
ולא בראוי כבמוחזק, לאיתויי נכסי דאבי אבא.
With regard to the repetition of the statement: Nor does he take twice the portion in property due the father as he does in property in his possession, this serves to include property of the father’s father, in a case where the father died before his own father. The firstborn does not receive his portion in such property, despite the fact that it will certainly come into the father’s possession, as this too is considered property that is due the father, not property in his possession. The same halakha applies also in the case of property due the wife, the daughters, or a yavam.
מתני׳ ואלו שאין חוזרין ביובל — הבכורה, והיורש את אשתו, והמייבם את אשת אחיו.
Mishnah: And these are the people whose properties, unlike an ancestral field, do not return to their original owners in the Jubilee Year: The firstborn who inherited his father’s property by the right of primogeniture need not return the extra portion for redistribution among the brothers; and one who inherits his wife’s property need not return it to her family; and one who consummates the levirate marriage with the wife of his brother and gains the right to his brother’s property need not return it for redistribution among the brothers.