Most households have multiple vehicles and some of them do buy more than one new vehicle in the same year. But more than likely they don't check your purchase history and simply send the offers to whoever is in the database.
Since rebates are specific to the purchaser (individual offers, local offers by zip code, etc.) and the vehicle (VIN#) the dealer simply puts your info and the VIN # into the system and it tells the dealer which offers are applicable to that purchaser and vehicle specifically, and that's what the dealer is allowed to submit.