Duncan is talking about the thane of Cawdor. Why might his words apply equally to Macbeth?
Ross has just finished telling Duncan how the thane of Cawdor teamed with the Norwegian forces against Scotland, and how Macbeth and Banquo beat them back. Duncan says, "No more that thane of Cawdor shall deceive / Our bosom interest: go pronounce his present death." "Our bosom interest" here is the safety of Scotland. This is ironic, since Macbeth will almost instantly deceive another bosom interest--Duncan's life itself.
Later, when Malcolm reports that before the treasonous thane of Cawdor was executed, he "very frankly...confess'd his treasons / Implored your highness' pardon and set forth / A deep repentance." Duncan responds, "He was a gentleman on whom I built / An absolute trust." Then he immediately turns to Macbeth and says, "O worthiest cousin!"
More irony, in that while Macbeth is worthy at this point, he will soon betray Duncan's "absolute trust."
(Malcolm's reports of the thane of Cawdor, however, don't apply to Macbeth, for when Macbeth meets Macduff in the final battle and learns that he was "from his mother's womb untimely ripp'd," he tries to surrender, but Macduff points out that he'd be set up to be an object of derision, which Macbeth cannot bear. Macbeth then says, "I will not yield, / To kiss the ground before young Malcolm's feet, / And to be baited with the rabble's curse." This makes him quite the opposite of the thane of Cawdor.)