Macbeth begins with thunder and lightning and the witches chant:"Fair is foul, and foul is fair".(I.i.10). Without being blatantly obvious, there is no doubt in the mind of the audience that Macbeth will hold several inconsistencies, many of them "foul."
Doublespeak understates events and infers ambiguity; interpretation is open to the audience and the characters; such as Macbeth's version of the witches prophesies. Banquo and Macbeth both hear the prophesies and both have much to gain but Banquo's interpretation of the "truths" that "the instruments of darkness tell us" (I.iii.124) and Macbeth's "cannot be ill; cannot be good"(131) result in quite different actions. Macbeth will take it upon himself to ensure that his "black and deep desires" (I.iv.51) are realized.
Macbeth's overriding confidence in the prophesies will lead to his ultimate downfall as he believes he is invincible especially when, on turning to the witches later in the play, they tell Macbeth that "none of woman born" (IV.i.80) shall harm him and that Macbeth shall"never vanquish'd be " until Birnam Wood advances on him about which Macbeth scoffs arrogantly and turns his thoughts to Banquo's sons who remain a concern in his fight "to reign in this kingdom."(102)
The duality in Macbeth is seen in the battle between good and evil. Macbeth is praised initially and as a noble warrior is rewarded by his king. It seems that Macbeth and Lady Macbeth's "vaulting ambition" will override all else but Lady Macbeth's increasing delusions and descent into madness as "more needs she the divine than the physician (V.i.72) and Macbeth's increasing risk-taking until he realizes that the witches are nothing more than "juggling fiends"(V.viii.19), serve to return the status quo as, in conclusion, "the dead butcher , and his fiend-like queen"(69) have been removed and the rightful heir will be "crown'd at Scone. "(75)
Hence, the doublespeak and duality reinforce the plot and ensure that the play ends fitfully and to the satisfaction of the audience.