Yes I did.

There's a islam-awareness thing going on around campus, and I was speaking to this guy at the booth, and what he had to say was interesting, but I couldn't help but notice one thing: he kept repeating "But you already know this, don't you?"

Which struck me as curious, since I obviously didn't, otherwise I wouldn't be asking And yet, there was something in that sentence that just made me wonder, Why? Why did he keep asking that? OK, maybe I should go back and ask him why. Would that be rude? Or inappropriate?

And do I want to know? Maybe it's just something he says. Or maybe it's some dark tactic to mess with your subconscious. To make you go "Well, OF COURSE, I knew it already".

If I suppose good intentions, and assume that this question is asked out of modesty, then it kinda makes sense. As in "I'm giving you all this info, but perhaps you already know all of this? Am I boring you?" And by doing this, you assume the one you're talking to is not an utter moron. Though, it only works when the person really did know it already.
Because it really could go the opposite way. As in "You know this already, don't you? Because I assume you do, because you really should be knowing all this stuff. Moron."

And if I assume bad intentions, and it I were the least bit paranoid, I'd be thinking he's trying to subconsciously force these facts into my mind. As in: "I'm saying something totally unrealistic and untrue, but you did believe that too, didn't you? Yes you did."