I am currently reading Harry Potter and the Half-Blood Prince and a question occurred to me. According to the story of the Deathly Hallows, the invisibility cloak is the third one. But in the previous books, we saw that many wizards own an invisibility cloak (like Mundungus, for example). How is that possible? As it is one of the three Deathly Hallows, shouldn't it be unique?
As I said, I've not finished the books, only the movies. Therefore, this question may be answered in the next book but still.. I would like to know the answer as there isn't any explanation in the movies.