Or HaChaim on Deuteronomy 1:8/אור החיים על דברים א׳:ח׳

1 ראה נתתי לפניכם את הארץ, "Behold, I have set the land before you;" The Torah switches from the singular ראה, to the plural לפניכם, "before you (pl), because when it comes to looking at the land they were all alike. However each individual Jew was different from his brethren in character, mentality, etc. Similarly, the nature of taking possession of the land and what it meant to each individual Israelite differed, so that the balance of the verse is in the plural.

2באו ורשו את הארץ, "Enter and take possession of the land, etc." Why did the Torah have to repeat the words את הארץ, instead of merely saying אותה, "it?" The reason is that the land which had been mentioned previously was the land which used to belong to Sichon and Og which G'd had allocated to Reuven and Gad. Now that Moses gave instructions to take possession of the land he had to make plain that the land which G'd had sworn to the Patriarchs had not included the lands now inherited by Reuven and Gad. I have already explained this in connection with Numbers 32,7 and have proved the point from the Sifri there.