Coding help

In my game there is a bookshelf that you are supposed to click on one of the books to guess where the clue is hiding. But whenever I click on one of the books it adds a new list item to the list I made. Then I have multiple list items but can only have three maximum because then it goes to a different part of my coding game. But I only want it to add it to the list once. What code do I need to make that?

There's a CONTAINS block in the list primitives that might help.