This is a recently popular expression of agreement that carries a subtext. It is more than agreement with an idea (observation, opinion); it is a friendly assertion that the speaker has already had the same idea–as in, "yeah, I know"–but it goes further: it seeks validation for the speaker's claim of precedence, from the very person whose claim he's jumping. The subtext might be "Oh, you took my idea; you should be agreeing with ME, not vice versa."
Student: I couldn't pay attention to the lecture because of that ball of sweat hanging from the professor's nose.
Classmate: I KNOW, right?