Doppelgangers, or doubles, are used in order to draw attention to certain qualities of each character. For example, a comparison between Macbeth and Banquo reveals how much more perceptive Banquo is than Macbeth, who appears quite easily manipulated and somewhat credulous. After their initial meeting with the Weird Sisters, Banquo is skeptical, questioning the motives of the witches and advising his friend to consider what they said with cautious. He says,
. . . oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray 's
In deepest consequence. (1.3.135-138)
In other words, he suggests that the sisters could have malevolent intentions and have told Macbeth a small truth—that he would become Thane of Cawdor—in order to betray him, trick him, into believing that he would become king, perhaps to see what he might do in order to make it come true. However, despite his friend's caution, Macbeth is already considering the "imperial theme" the witches promised and how it might come about (1.3.142).
A comparison of Lady Macbeth and the Weird Sisters shows how much they have in common. They both seem to manipulate Macbeth for their own ends, and they both advocate deception. In the very first scene of the play, the Weird Sisters say,
Fair is foul, and foul is fair;
Hover through the fog and filthy air (1.1.13-14)
This makes is seem as though, through their manipulations, good things will begin to look bad and bad things will begin to look good, and the deceptiveness of appearances will fool people into doing things they might not normally. Then, when Macbeth arrives home, Lady Macbeth tells him to
Look like th' innocent
But be the serpent under 't. (1.6.76-78)
In other words, she wants him to look kind and welcoming when Duncan arrives in order to hide his dark intentions to murder the king that night. Ultimately, both Lady Macbeth and the Weird Sisters let Macbeth down, too. His wife takes her own life, likely as a result of her guilty conscience, leaving Macbeth all alone and friendless in the world, and the sisters trick him into believing that he is invulnerable so that he only finds out too late that he isn't.
Now, can you compare and contrast Macbeth and Macduff? What qualities do they share, if any? In what ways are they opposites?