What do duncan's comments about the late thane of cawdor reveal?

Macbeth questions whether "the devil speaks true." He is referring to the witches and one of their predictions of him being the thane of Cawdor. The prediction is true yet MacBeth knows the Thane of Cawdor to be alive. Angus informs MacBeth that the thane is dead. MacBeth continues in his aside to wonder about the other prophecy of becoming king.