I assume you are refering to the end of this chilling story, when Montressor has chained up Fortunato and converses with his enemy behind the wall that he has just constructed to brick him in. It seems that the repetition of Montressor, as he eerily repeats the words of Fortunato, only serves to highlight the horror of his act in chaining his enemy to the wall and then bricking him in to die. Note what is said between them:
"He! he! he!--he! he! he!--yes, the Amontillado. But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone."
"Yes," I said, "let us be gone."
"For the love of God, Montressor!"
"Yes," I said, "for the love of God!"
Note how the repetition seems to function only to augment the horror of Montressor and his actions. This is yet another indication that would suggest that Montressor is mad and a completely unreliable narrator. He seems to delight in tormenting his victim and torturing him with his words at this stage in the narrative, showing his true colours.