It has quite the unique ability, as it will only activate if you have no duplicate in your deck (2 of the same cards). This does not include cards in your hand, on your board, or cards used. So purely what's currently in your deck.
And when you have that requirement, Librarian will add 3 random cards to your deck. This can be anything, from spells of other classes to Determination cards. But it can give multiple of the same card as well as cards you already have in your deck, so be careful.
Afterwards, you will draw 2 cards. So be sure you have a low amount of cards in your hand before using this.