I can't write this paper for you, but I can provide you with some direction. This is obviously an opinion statement that you are being asked to support, so here goes!
Duncan does not appear to have the ability to "read" people or to see through the facades that people close to him put up. An example is the original Thane of Cawdor who turns out to be a traitor. He says that this Thane "was a gentleman on whom I built an absolute trust." Later, when he arrives at Macbeth's castle, he feels instantly at ease. Because of the constant attempts on their lives and for their thrones, Kings had to be excellent judges of character for their own preservation and for that of their families.
Secondly, even if Duncan cannot recognize false people when he sees them, a king certainly would have those closest to him under constant surveillance. After all, one can be most hurt by those closest to him. However, the Thane of Cawdor is able to aid the Norwegian army without being discovered beforehand. As a result, the entire country is in peril.