Duplicity of character and motive are most evident in those captivated by the witches' prophesies: ie. Macbeth and Lady Macbeth. As the witches incantation at the end of Act 1 Scene 1 tells us-
Fair is foul, and foul is fair.
Hover through the fog and filthy air.
Good becomes bad - as 'brave Macbeth' descends into mindless slaughter.
Macbeth and Banquo are bewildered by the words of the witches, and when Macbeth muses upon them, he is still confused
This supernatural soliciting
Cannot be ill, cannot be good.
Once made aware of the prophesies, Lady Macbeth decides that both her and her husband will need to hide their true motives and ambitions for them to succeed -
look like the innocent flower,
But be the serpent under't
The Macbeth's work under the shadows of duplicity and deception even when Macbeth has the throne. The irony is in the duplicity that the witches have duped Macbeth with in the form of the prophesies. Birnam Wood does indeed move to Dunsinane Hill as the soldiers disguise themselves to storm the castle. Macduff, not of woman born, is able to slay Macbeth as he is of Caesarian birth.
What seemed a 'fair' future for the Macbeths becomes most 'foul' by the end of the play.
Shakespeare is a master at doubling, duality, and duplicity in characters, themes, and imagery. For nearly every major character, there is a double (a foil or doppelganger). The biggest duality is Macbeth himself; he is the prototypical hero turned villain. His doppelganger (ghostly twin) is Banquo. Nearly everyone is his foil. His nemesis is Macduff. The play is a series of revenges: man vs. man, man vs. supernatural, man vs. self, even nature vs. man.
Here's a rough list of them:
Macbeth vs. self (hero turned villain)
Macbeth vs. McDonwald (honorable vs. traitorous)
Macbeth vs. Banquo (two heroic thanes who differ in response to witches' prophecies)
Macbeth vs. Duncan (the honorable vs. the soon to be dishonorable)
Macbeth vs. Lady Macbeth (murderous hosts; unnatural; childless)
Macbeth vs. Macduff (villain vs. hero)
Macbeth vs. Banquo's ghost (man vs. spirit)
Macbeth vs. Malcolm (evil may give way to evil)
the Macbeths vs. the Macduffs (unnatural, childless family vs. natural, child)
dagger vs. imaginary dagger
madness vs. sanity
natural vs. unnatural vs. supernatural
light vs. dark
child vs. childless
bloody vs. clean (water)
appearance vs. reality
nature (Birnham Wood) vs. man (Macbeth)