I could see your lips move
HAL: I know that you and Frank were planning to disconnect me. And I'm afraid that's something I cannot allow to happen.
Dave Bowman: Where the hell did you get that idea, HAL?
HAL: Dave, although you took very thorough precautions in the pod against my hearing you, I could see your lips move.
Context: HAL reveals how it learned of the astronauts' private plan.
Not in a duel yet