One blessing is sufficient, and blessings should not be treated in a superstitious fashion (e.g., getting an item blessed multiple times to “make sure” it’s done properly). But an item can be blessed again for just reason. For example, a pet can be taken to a yearly blessing of animals on the feast of St. Francis; a formerly blessed item that is sold to a new owner—and thus loses its blessing—can be reblessed for the new owner (e.g., a house). And, yes, if you’re not sure whether or not something has ever been blessed, you can have it blessed.
