In Citadels it's possible to build both the Library and Observatory:
If you choose to draw cards when you take an action, you keep both of the cards you have drawn.
If you choose to draw cards when you take an action, you draw three cards, keep one of your choice, and put the other two on the bottom of the deck.
What happens when I draw district cards when I have both districts in my city? Do I draw three cards and keep all of them, or do I draw three cards and keep two of them?