What is the importance of Amir’s dream of Assef claiming to be his twin? Why would Hosseini include this dream in the story of Amir’s recovery?

Amir feels crushing guilt about his ill-treatment of Hassan. In a way, Amir thinks that his own darkness is a watered-down version of Assef. The hero/villain motif in the movies they used to watch is at play here: both are similar people who just take different paths. Hosseini includes this dream to show that Amir is maturing and becoming more self-aware.