Having already taught us that all fields revert to their original owners in the Yovel (See Pasuk 10), why does the Torah repeat it here?
Rashi: To include a case where the seller's son redeems the field that his father sold - which also reverts to his father in the Yovel.
Seforno: The Torah is teaching us here that the seller is permitted to establish ownership over the field [in Yovel] to build houses, dove-cotes and sheep-pens, as long as does not work the land or perform acts that guard the fruit.
QUESTIONS ON RASHI
Rashi writes that the repetition includes a son who redeemed what his father sold. A Kal v'Chomer teaches this - what others bought returns, all the more so what his son bought! If one was Makdish his Sedei Achuzah, it returns to him if his son redeemed it from Hekdesh, but not if someone else did (Erchin 25b)!
Moshav Zekenim: Buying from Hekdesh and from a buyer are different. One might have thought that if the son buys from others, he keeps it, lest people refrain and not redeem. Even though in any case it would return in Yovel, it is disgraceful for others to keep it until Yovel. R. Akiva (Kidushin 15b) says that if relatives redeem one who sold himself to Nochrim, he serves them. If a stranger redeems him, he goes free. However, the Gemara says that he serves relatives, lest one be quick to sell himself to a Nochri, confident that his relatives will redeem him! Rather, according to R. Akiva 1 , one might have thought to fine the father, lest he sell and rely on relatives to redeem.
Riva: The Torah wrote it, even though we could learn it from a Kal v'Chomer.