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 4 random cards to your deck. This can be anything, from spells of other classes to Determination cards. But it will not be a card you already have in your deck, so you can use another Librarian later on.
Afterwards, you will draw 3 cards. So be sure you have a low amount of cards in your hand before using this. Also keep in mind that you need to draw 2 of the 3 Spiders given by Muffet before using this, otherwise its effect won't trigger.