A stock response to anything glaringly obvious and/or ridiculously paranoid that somebody high might say. Sounds genuine to a stoner but has a mocking undertone from which sober people can draw amusement. Derived from an episode of Sealab 2021, a stoner favorite.
Sparks: "Marduk, you totally rule"
Marduk: "I totally already know that"
Stoner: "McDonalds is totally responsible for like, two point eight billion deaths from starvation in Canada alone"
Sober guy: "I totally already know that."