Doublespeak and duality apply to the prophecies that Macbeth received from the witches about danger.
Macbeth interprets the witches’ prophecies as demonstrating that he is not in danger. His reactions to these seal his doom. In his arrogance, he assumes he is safe.
A perfect example of this is the prophecy regarding Macduff. Macbeth is aware that Macduff is coming after him, in revenge for Macbeth’s ordering the murders of his entire family. Macbeth wants to know if Macbeth is a danger to him.
Be bloody, bold, and resolute; laugh to
The power of man, for none of woman born
Shall harm Macbeth. Descends.(90) (Act 4, Scene 1)
Macbeth’s reaction is “Then live, Macduff. What need I fear of thee?” He is convinced that Macduff is no danger to him, and the witches reinforced that idea. No man born of woman can be a danger to him, and since every man has to be born from a woman, he is safe.
Macbeth is still convinced of this after some of the other prophecies turn out to have double meanings. He was supposed to be safe until the forest moved to him, and yet the soldiers pretended to be trees and so the forest basically did.
When Macbeth meets Macduff in battle, he taunts him that he is safe. Macduff delivers a blow.
Despair thy charm,
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Untimely ripp'd.(20) (Act 5, Scene 8)
When Macbeth finds out that Macduff was born by C-section, he is scared. He loses his courage, and suddenly realizes that he is doomed. Psychologically, he is no longer able to fight. The feeling of safety being pulled out from under him deflates his arrogance, and he essentially submits to dying in battle.