The doppelganger is a literary technique in which a double (or look-alike) appears in the story as a representation of evil. In the case of "The Cask of Amontillado," there is no real tangible character of the doppelganger; instead, the usage appears in the form of the mocking tone in which Montresor repeats all of Fortunato's desperate cries at the end of the story. When the manacled Fortunato finally manages to respond to Montresor's surprising act of betrayal, he exclaims
Montresor's reply is "True... the Amontillado." When Fortunato is next heard, it is through "A succession of loud and shrill screams..." Montresor echoes him:
I replied to the yells of him who clamoured. I re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamourer grew still.
Montresor recapitulates Fortunato's words twice more.
"Let us be gone."
"Yes,” I said, “let us be gone."
"For the love of God, Montresor!"
"Yes," I said, "for the love of God!"