Why does the Torah repeat "vi'Yeshavtem la'Vetach alehah"?
Rashi: It is an assurance against drought. 1
Ramban: It is an assurance that there will be plenty to eat in the land, and that they will not need to go and live in other lands for lack of food.
Seforno: It is an assurance that there will be plenty to eat in the land, and that they will not have to suffer the embarrassment of having to go and purchase food from other nations.
See Sifsei Chachamim as to why Rashi inverts the two Diburim.
Having said "Ve'nasnah ha'Aretz Piryah", why does the Torah see fit to add "Va'achaltem Lachm'chem la'Sova'?
Rashi: To teach us that even in the stomach, the food will be blessed. 1
Seforno: It teaches us that the fruit was highly sustaining qualitatively, 2 in that the fruit of the sixth year sufficed for the seventh year as well.