The word “double” is a reference to the witches, and a reminder that they have had a hand in Macbeth’s downfall.
The word “double” is oft repeated throughout the play. Here it echoes the witches’ incantation, “Double, double, toil and trouble” (Act 4, Scene 1) in which they stirred up damage for him. The witches did not begin then. From the very beginning, they have been harbingers of doom setting the mood of the play. They represent the inner workings of Macbeth’s own twisted mind.
Macbeth responds with the comment about “these juggling fiends” when Macduff reveals that he was born by Cesarean. The witches told him he could not be killed by “man that's born of woman” (Act 5, Scene 3). Now he learns there is a little loophole in that logic. Technically, Macduff was not born of a woman because he came out unnaturally.
He feels that the witches have been messing with him all along. He starts to lose his nerve at this point. In fact, he spits at Macduff, commenting that he won’t fight him in one breath, but that he won’t give in yet in another.
I will not yield,
To kiss the ground before young Malcolm's feet,
And to be baited with the rabble's curse. (Act 5, Scene 8)
Macbeth has been making a downhill spiral for some time now. His wife’s death did not sit well with him. Seeing the witches’ new prophecies, including all of those Banquos that would be heir to his throne, did not make him feel good. So he is really frustrated and feels he has nothing to lose. When he meets Macduff on the battlefield he is not even really feeling the desire to fight to the death anymore. It is more of a “poor me” attitude. He has been everyone’s fool. He knows it is not much longer now, and Malcolm will be king.