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This phrase is like saying someone's a total idiot who's trying to mess with your head. It's used when someone's being a huge pain in the butt on online games.
You: Why are you spamming this chat? Them: Obvious troll is obvious.
Your friend: That guy's been clogging up the server for hours. You: Obvious troll is obvious.
In the comments: This guy's a waste of oxygen. Reply: Obvious troll is obvious.