Duncan Idaho is a fictional character in the Dune universe created by Frank Herbert. He is a recurring character from the science fiction novel Dune and its five sequels, the only character to do so.

At the time of Dune, Idaho is a swordmaster in the service of House Atreides, and one of Duke Leto's right-hand men (with Gurney Halleck and Thufir Hawat). When the Atreides take over Arrakis, Idaho becomes Leto's ambassador to the Fremen, the desert people of Dune that Leto hopes will ally with him in the coming war against the Emperor and the Harkonnens; Idaho goes to live with the Fremen, serving Stilgar and Leto both.

When the Emperor attacks Dune in the guise of Harkonnen troops, Idaho is killed saving Paul Atreides and Lady Jessica.

There is some suggestion that Duncan Idaho is the last living swordsman to have completed the training of House Ginaz, renowned for its sword schools. House Ginaz was destroyed in a war of assassins with House Moritani.

Idaho returns in Dune: Messiah as a ghola, a clone made by the Bene Tleilaxu and gifted to Paul Atreides, now emperor. The purpose of the gift is 'psychic poison': Idaho is supposed to tempt the Kwisatz Haderach into becoming that which he despises. Known as Hayt in his ghola incarnation, Idaho was trained as a mentat and a zensunni philosopher. He has no memory of his former life, but constantly feels vague hints and reminders of his previous incarnation that make him hope he can recover those memories.

Failing to destroy the emperor psychically, Hayt is programmed to kill Paul against his will. In the stress of the attempt, Hayt recovers his memories as Duncan Idaho, up to and including his death at the hands of Imperial Sardaukar; he stops the attempt, and becomes, once more, a loyal Atreides servant, incidentally demonstrating to the Tleilaxu what they had been searching for: a way to link one incarnation of a ghola to all of them in memory, which provides a kind of immortality, and a bank of wisdom similar to what the Bene Gesserit have in other memory.

In Children of Dune, Idaho marries Alia, regent to Paul's children Leto II and Ghanima. Alia becomes possessed by the memory-ego of her ancestor, the Baron Vladimir Harkonnen, who sets out to destroy the Atreides empire. Recognizing this, Idaho remains loyal to the Atreides, and flees to the desert to protect the children. In order to get Stilgar to join the Fremen opposition to Alia's rule, Duncan kills Javid, Alia's secret lover, on the neutral ground of Sietch Tabr. He also insults Stilgar three times, telling him "You wear a collar." Driven into a rage, Stilgar kills Duncan for violating the sietch's neutrality.

God Emperor of Dune reintroduces Idaho as a serial ghola: Leto has ruled as a sandworm for 3,500 years, and has continually had an Idaho ghola to serve him. To Leto, Idaho represents something fundamentally vital about humans, and an exemplar of loyalty. However, the Idaho gholas, with their memories restored, struggle to deal with what the Atreides have become. Where before, the hallmark of Atreides' rule was justice, Leto's reign has been one of godhood and oppression, and Idaho's old-fashioned conscience usually rebels, leading the Idaho ghola to attempt to kill Leto (who is nearly invulnerable as a sandworm).

The Bene Gesserit become the consumers of Idaho gholas in Heretics of Dune and Chapterhouse: Dune. However, the Tleilaxu keep killing the gholas after several years. Not knowing exactly what purpose Idaho will serve, the Bene Gesserit suspect that the Tleilaxu are using the gholas to control the timing of his release upon the universe, implying some Tleilaxu purpose in addition to their own. The current ghola survives, and entraps an Honored Matre, revealing the Tleilaxu purpose: The Honored Matres, who are all female, rule by the sexual enslavement of men--their sexual abilities dwarf what the Bene Gesserit can accomplish, to the point of creating physical addiction in men; Idaho is the Tleilaxu response. When Murbella, an Honored Matre, tries to sexually bond Idaho, she is also enslaved, and the two of them are confined to the no-ship on Chapterhouse. There, Idaho trains young men to go out into the universe and enslave Honored Matres. Miles Teg was commissioned to restore the memories of Duncan Idaho and does so, but Duncan believes the Tleilaxu planted something else in his mind, he can feel it. When Murbella tries to sexually bond Duncan the Tleilaxu thing occurs, all of the other Duncan Idaho gholas flood into his awareness like links of sausages, Duncan gains ALL of his memories, original and serial ghola. Duncan also inherits an awkard "prescient" type vision, where he sees a man and a woman staring back at him.