There are at least four reasons why Montresor keeps repeating, "I have my doubts."

• He pretends that he needs to have a connoisseur sample the cask of Amontillado he has just purchased. He would have to have doubts about its authenticity if he wanted to be assured by an expert.
• He implies by his words and actions that he would like to buy more of the wine at the "bargain" price he mentions if he were sure it was genuine Amontillado. Also, that he is in a big hurry to buy more before word of the "bargain" gets around.
• He cannot be sure that Fortunato will come with him that night. If not, then Fortunato will certainly inquire about the wine the next day or at least very soon. Since Montresor has no Amontillado, he plans to bring Fortunato a bottle of ordinary sherry and claim this comes from the cask he just purchased. Fortunato will taste it, realize it is not genuine Amontillado, and forget about the matter. Montresor will have to think of some new scheme for killing Fortunato.
• Montresor knows that Fortunato would cheat him if he really had a cask of Amontillado. Fortunato would judge it to be ordinary sherry, then go and find the ship that had just brought the Amontillado into the harbor and buy up the entire cargo. By repeating, "I have my doubts," Montresor is suggesting that he could easily be discouraged from trying to buy any more of the wine. He is already half-convinced that it is not genuine. Neither man is interested in drinking 126 gallons, or 504 quarts, of Amontillado. They both regard it as an investment. "I have my doubts" is an open invitation to Fortunato to trick him.

What song the Syrens sang, or what name Achilles assumed when he hid himself among women, although puzzling questions are not beyond all conjecture.
                                              Sir Thomas Browne, “Urn-Burial.”

In a famous review of Nathaniel Hawthorne's Twice-Told Tales, Poe defined the short story as follows:

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. And by such means, with such care and skill, a picture is at length painted which leaves in the mind of him who contemplates it with a kindred art, a sense of the fullest satisfaction. The idea of the tale has been presented unblemished, because undisturbed; and this is an end unattainable by the novel. Undue brevity is just as exceptionable here as in the poem; but undue length is yet more to be avoided.

Poe expected the reader of "The Cask of Amontillado" to make many deductions, assumptions and inferences from the information Montresor provides. This is something new in fiction writing. It seems to foreshadow Ernest Hemingway's "iceberg theory."

If a writer of prose knows enough of what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of an ice-berg is due to only one-eighth of it being above water. A writer who omits things because he does not know them only makes hollow places in his writing.
                             Ernest Hemingway, Death in the Afternoon

There is a great deal hidden beneath the surface in Poe's masterpiece. Much of this is deducible or inferable to "the mind of him who contemplates it with a kindred art." Poe understood what Hemingway was talking about—and much more—long before Hemingway was born.

For example, Montresor twice tells Fortunato, "I have my doubts." He has already bought and paid for the pipe of wine, i.e., 126 gallons. Why does he need to satisfy his doubts after the fact? The obvious deduction from his speech and body language is that he wants to buy more before word gets around that a shipment is being offered at a "bargain" price. That is why he pretends he is going to see Luchesi, since he pretends he was unable to find Fortunato. Montresor never had any intention of going to Luchesi; he only uses Luchesi’s name to compel Fortunato to come immediately. Fortunato does not need to go to Montresor’s catacombs at all. He only goes to keep him from going to Luchesi. 

Furthermore, by telling Fortunato he has his doubts about the wine, Montresor is providing his friendly enemy with the notion that it would be easy to cheat him. Montresor has suffered a "thousand injuries" at the hands of Fortunato and knows him well. He knows that if the wine really existed, Fortunato would sample it, lick his lips, frown, shake his head, and declare that it was only ordinary sherry. Then, assuming it was genuine Amontillado, this connoisseur would rush off to find the seller. It would have to be someone newly arrived aboard a ship from Barcelona. Everyone on the docks could point it out to him. Fortunato is a rich man. Unlike Montresor, he could buy up the entire cargo and make a fortune bottling and reselling it at his own convenience. Amontillado in an oak casket will not only keep indefinitely but will improve in taste and value with age. In terms of current American dollars, Fortunato is visualizing a profit of something like $100,000! When Montresor found out he had been duped, Fortunato would laugh and call it “an excellent jest.”

There is another reason which the person who contemplates the story with a kindred art should be able to deduce. If for some reason Fortunato cannot or will not accompany Montresor to his palazzo that night, Fortunato will still want to taste this gourmet sherry at the earliest opportunity. Since the Amontillado does not really exist, as we ourselves realize later on, Montresor will have to bring Fortunato a bottle of ordinary sherry and claim that this came from the pipe--which means that he will have to concoct a whole new way of killing his hated enemy with impunity. Fortunato will taste the sample and know it is not genuine Amontillado. He will assume Montresor has been cheated and will lose all further interest. Fortunato would undoubtedly learn that there was no Spanish ship in the harbor with a cargo of sherry, but Montresor could claim that the cask was delivered to him and he never saw where it came from. This could help explain why he has his doubts.

The statement "I have my doubts" is just one of dozens of clues to understanding the story, particularly to understanding how Montresor planned and executed the perfect crime he confesses to in his narrative. Every detail that raises questions in the reader's mind can be explained, not merely as plausible, but as essential to the author's creation of "a certain unique or single effect." The words "single effect" define the modern short story. Poe means a feeling, an emotional effect. That effect can come at the very ending, as in an O. Henry story, or it can be an effect that is created by the entire mood and tone. "The Cask of Amontillado," with its chilling ending, is a perfect illustration of Poe's rationale.

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.

In Edgar Allan Poe's "The Cask of Amontillado," why would the Montresor repeat that he "has his doubts" about the Amontillado he purchased?

In a famous review of Nathaniel Hawthorne's Twice-Told Tales, Poe defined the short story as follows:

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. And by such means, with such care and skill, a picture is at length painted which leaves in the mind of him who contemplates it with a kindred art, a sense of the fullest satisfaction. The idea of the tale has been presented unblemished, because undisturbed; and this is an end unattainable by the novel. Undue brevity is just as exceptionable here as in the poem; but undue length is yet more to be avoided.

Poe expected the reader of "The Cask of Amontillado" to make many deductions from the information his narrator Montresor provides. This is something new in fiction writing. It seems to foreshadow Ernest Hemingway's "iceberg theory." There is a great deal hidden beneath the surface in Poe's masterpiece. Much of this is deducible to "the mind of him who contemplates it with a kindred art."

For example, Montresor twice tells Fortunato, "I have my doubts." For one thing, this suggests that he is anxious to buy more of the Amontillado at the bargain price he mentions but that he has to be assured it is genuine. He has already bought and paid for the pipe of wine, i.e., 126 gallons. Why does he need to satisfy his doubts after the fact? The obvious deduction from his speech and body language is that he wants to buy more of the wine before word gets around that a shipment is being offered at a bargain price. That is why he pretends he is going to see Luchesi, since he pretends he was unable to find Fortunato.

Furthermore, by telling Fortunato he has his doubts about the wine, he is providing his friendly enemy with the notion that it would be easy to cheat him, to discourage him from buying more of it. Montresor has suffered a "thousand injuries" at the hands of Fortunato and knows him very well. He knows that if the wine really existed, Fortunato would sample it, lick his lips, frown, shake his head, and finally declare that it is only ordinary sherry. Then, assuming it was genuine Amontillado, this connoisseur would rush off to find the seller. It would have to be someone aboard a newly arrived ship from Barcelona, which would be easy for a man with Fortunato's experience to do. Fortunato is a rich man. He could buy up the entire cargo and make a fortune bottling and reselling it. When Montresor found out about it, he would laugh and call it an excellent jest.

There is another reason which the person who contemplates the story with a kindred art should be able to deduce. If for some reason Fortunato cannot accompany Montresor to his palazzo that night, Fortunato will still want to taste this gourmet sherry at the earliest opportunity. Since the Amontillado does not really exist, as we ourselves realize later on, Montresor will have to bring Fortunato a bottle of ordinary sherry and claim that this came from the pipe--which means that he will have to concoct a whole new way of killing his hated enemy with impunity. Fortunato will taste the sample and know it is not genuine. He will assume Montresor has been cheated and will lose all further interest. Fortunato would undoubtedly learn that there was no Spanish ship in the harbor with a cargo of sherry, but Montresor could claim that the cask was delivered to him and he never saw where it came from. This supposed fact could partially explain why he has his doubts.

The statement "I have my doubts" is just one of dozens of clues to understanding the story, particularly to understanding how Montresor planned and executed the perfect crime he confesses to in his narrative. Every detail that raises questions in the reader's mind can be explained, not merely as plausible, but as essential to the author's creation of "a certain unique or single effect." The words "single effect" define the modern short story. Poe means a feeling, an emotional effect. "The Cask of Amontillado," with its chilling ending, is a perfect illustration of Poe's rationale.

In Edgar Allan Poe's "The Cask of Amontillado," why would the Montresor repeat that he "has his doubts" about the Amontillado he purchased?

Montresor tells Fortunato that he has bought a pipe of what has been represented as Amontillado but he has doubts about its authenticity and is seeking an expert opinion, either that of Fortunato or of Luchesi. Montresor has already bought and paid for the wine, yet he appears to be in a big hurry to find out whether it is genuine. Why? He mentions that he bought it hurriedly because it was a bargain. Obviously what he is trying to convey by his story and his apparent haste to get a verification is that he would buy more if only he could be sure it is genuine Amontillado. Since he already owns 126 gallons in the first pipe, he obviously does not want the wine for personal consumption but for resale at a profit. One attractive feature of wine such as Amontillado Sherry in an oak cask is that it will only improve with age. He could take his time about bottling and selling the wine (assuming he really had it at all), and possibly charge more as the wine aged. Montresor is a poor man and might only be able to buy a few more pipes--but Fortunato is a rich man and could buy the entire cargo, thereby getting an even better bargain.

It is the bargain price that interests Fortunato--and Montresor knows it. Fortunato is not the type of man who would go out of his way to sample the wine just to help a friend. He is not anxious to travel a long distance on foot and then creep through underground passages just to drink a glass of Amontillado, however delicious. He is not all that anxious to show off his "virtuosity" as a wine connoisseur. He visualizes a ship from Barcelona loaded with casks of Amontillado available at a bargain price. He thinks only Montresor knows about this bargain right now, but once word gets around there will be more buyers driving the price up. Fortunato especially does not want Montresor to go to Luchesi, because Luchesi knows wine and probably has the money to buy up the entire cargo himself. That is the only reason Fortunato goes to Montresor's palazzo. Otherwise, he could find the ship by himself, sample the wine on board, and deal with the seller directly. Montresor knows that Fortunato has no intention of helping him because he knows his man. He has been injured by him a thousand times already. He knows that Fortunato would-- (if the wine really existed)--take a sip, frown, take another sip, shake his head, and pronounce it ordinary Sherry. This would eliminate Montresor as a competitive bidder for the remaining Amontillado aboard the Spanish ship. Montresor knows that his fictitious Amontillado is genuine because he invented it, but he also knows that Fortunato would consider it an "excellent jest" to judge it as ordinary Sherry and then buy the remaining cargo out from under Montresor. Fortunato, without even tasting the wine, has told Montresor, "You have been imposed upon," thereby preparing him for disappointment.

In Edgar Allan Poe's "The Cask of Amontillado," why would the Montresor repeat that he "has his doubts" about the Amontillado he purchased?

Montresor, the protagonist of Edgar Allan Poe's "The Cask of Amontillado," is seeking revenge against Fortunado for the "thousand injuries" he placed upon Monstrsor. As part of his plan for revenge, Montresor must get Fortunato into the catacombs beneath his family's home. 

In order to do this, Montresor tells Fortunato that he possesses a cask of Amontillado. Since Amontillado is very had to come by, Montresor is able to use his concern regarding the authenticity of the wine. By repeatedly questioning the authenticity, Montresor is able to keep Fortunato's interest in it peaked. The more Montersor questions the authenticity, the more Fortunato wishes to test the wine. This is essential to getting Fortunato into the catacombs so that Montersor can see his plan of revenge through.