In 7:1-3 the Torah commands us to wipe out the nations already residing in the Land of Israel. It then commands us to not allow them to continue living there, and to not intermarry with them....
If we already have to wipe them out, then why is it necessary to have secondary commandments of not letting them buy property and not intermarrying?
It is true that if we would fulfill our obligation of wiping out these nations, then the secondary commandments will have become moot and pointless. The problem is if we do not fulfill our obligations and we do not wipe them out completely. The Torah seems to be predicting that we will not fulfill ourobligations so it is letting us know what the natural results of such inaction will be.
If we let them stay, they will end up demanding rights. They will buy property, they will integrate. They will become a part of society. If we allow them to remain, natural relationships will develop.
Therefore the Torah has to command us to not allow those relationships.
By not fulfilling the first part of the mitzva, the second part becomes necessary.