“I already know that” is the most expensive sentence a student can say.

Knowing about something and knowing it are different states, and the first one feels almost identical to the second from the inside. That sentence closes the door before the room has been checked. What it protects is not your time but your self-image, and the cost lands later, when the thing you skimmed turns out to be the thing you needed.

Swap it for a question the next time it rises. Ask what the person would add that you have not heard, or try explaining the topic aloud and see where it thins out. Familiarity is not the same as command.