Early in "The Cask of Amontillado," Montresor repeats the line "I have my doubts" to Fortunato. In your opinion, why does he do this?

When Montresor initially meets Fortunato in the streets, he behaves amicably, shaking his enemy's hand, and he mentions that he believes he has acquired a pipe of rare Amontillado wine. Montresor plans on convincing Fortunato to follow him into his family's catacombs and must give a reasonable explanation that would persuade his enemy to follow him. Montresor understands that he must show uncertainty in order to suggest that Fortunato should sample and identify the wine, which is why he says, "I have my doubts." As Fortunato proceeds to mention that it is impossible for Montresor to acquire such rare wine during the carnival season, Montresor once again says, "I have my doubts," in order make his predicament believable. Fortunato has enough knowledge to realize that Montresor may be lying about possessing the rare wine, which is why Montresor must "play dumb" and appear to be naive and inexperienced. Montresor proceeds to explain that he did not want to lose out on a bargain and made the snap decision to purchase the Amontillado without consulting Fortunato. When Fortunato once again expresses his disbelief, Montresor repeats, "I have my doubts," for the third time. Overall, Montresor continues to repeat that he has his doubts regarding the wine's authenticity to make his situation believable and to persuade Fortunato into following him down into his family's extensive vaults to taste the rare Amontillado.

When Fortunato exclaims, "'Amontillado?  A pipe?  Impossible!'" Montresor seems to admit to Fortunato that he also has some doubts about whether or not the pipe of Amontillado he claims to have purchased could be genuine.  Montresor has already told us that Fortunato absolutely prides himself on many things, including his discernment as a connoisseur of wine.  It seems to me that Montresor's pride will not allow him to totally paint himself as the gullible buyer of a product he likewise knows to be impossible.  He claims that he has doubts about the wine's authenticity in order to assuage his disgust at having to play dumb for a man he hates.  Even to pretend to need Fortunato's assistance seems to be distasteful to Montresor, and his repetition of the line, "'I have my doubts,'" indicates his inability to completely defer to Fortunato's judgment, even in service of his plan.