Why does the Torah repeat "Vehorashtem" again?
Rashi: This is not a Mitzvah but a promise that if they destroy the inhabitants of the land, they will be able to settle it and remain there, otherwise not!
Ramban: This Pasuk is presenting a Mitzvah to dwell in Eretz Yisrael and possess it - Should one decide to go and conquer Bavel, Ashur or any other country and to live there, one will contravene an Asei. 1
Seforno: This is a promise that if they destroy the inhabitants of the land, they will be able to pass it on to their heirs as an inheritance, otherwise not! 2
Ramban: And when the Gemara in Kesuvos 110, stresses the obligation to reside in Eretz Yisrael, placing a prohibition against leaving it and labels a woman or a man who refuses to move there with respective spouses 'a Moredes' (a rebel), it does so based on this Pasuk.
Seforno: Despite the fact that they themselves will succeed in conquering it.