Duncan Idaho is a fictional character in the Dune universe created by Frank Herbert. He was introduced in the first novel of the series, 1965's Dune, and became a breakout character. He was revived in 1969's Dune Messiah. He is the only character to feature in all six of Herbert's original Dune novels.

Idaho was portrayed by Richard Jordan in David Lynch's 1984 film version of Dune, and by James Watson in the 2000 Sci-Fi Channel miniseries. Edward Atterton assumed the role in the 2003 miniseries Children of Dune. The character is played by Jason Momoa in the 2021 Denis Villeneuve film Dune and the upcoming third installment, Dune: Part Three (2026).

In Dune (1965), Duncan is described as a handsome man with "curling black hair" to whom women are easily attracted. Paul Atreides notes Duncan's "dark round face" and "feline movements, the swiftness of reflex that made him such a difficult weapons teacher to emulate." And in Dune Messiah (1969) he is described as having "high cheekbones" and "definite epicanthic folds." Lady Jessica calls him "the admirable fighting man whose abilities at guarding and surveillance are so esteemed." Duncan is fiercely loyal to House Atreides, a skilled pilot, and as a Swordmaster of the Ginaz is a gifted hand-to-hand fighter. In the fight that ends with his death in Dune, Duncan kills an unheard-of 19 Sardaukar, the Padishah Emperor's fearsome supersoldiers. The Sardaukar sell his body to the Tleilaxu; subsequent gholas of Duncan possess the rebellious streak of the original.

In 1976's Children of Dune, Lady Jessica tells Duncan that he was drawn to her daughter Alia because "you wanted a girl you saw as a younger version of me." In God Emperor of Dune (1981), the latest Duncan ghola discovers that one of his predecessors had fathered a child with a woman named Irti who closely resembles Jessica.

At the time of Frank Herbert's original novel Dune (1965), Idaho is a Swordmaster of the Ginaz 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 the planet Arrakis at the order of the Padishah Emperor Shaddam IV, 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 both Leto and Fremen leader Stilgar. When the Emperor's dreaded Sardaukar attack Arrakis in the guise of Harkonnen troops, Idaho initially survives the assault and saves Leto's son Paul Atreides and concubine Lady Jessica. When they are cornered at a Botanical Testing Station, Paul and Jessica flee while Idaho holds off the enemy, but he is ultimately killed. It is noted in Dune Messiah (1969) that a "grievous head wound" had caused Idaho's death, and made clear in Children of Dune (1976) that Idaho had killed an unheard-of tally of nineteen Sardaukar before dying.

Idaho returns in Dune Messiah (1969) as a ghola named Hayt, made by the Tleilaxu and given to Paul Atreides, now Emperor. The purpose of the gift is 'psychic poison': the ghola is trained to tempt the Kwisatz Haderach Paul into becoming that which he despises. Trained as a Mentat and a Zensunni philosopher, Hayt 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.

The seemingly harmless dwarf Bijaz recognizes that Hayt is failing to destroy the Emperor psychologically. As the ghola is of Tleilaxu design, Bijaz employs a specific humming intonation that renders Hayt open to implanted commands. Bijaz programs Hayt to kill Paul when he says the words "She is gone," in the depths of grief over the death of his concubine Chani. But this is only part of the true plan; the emotional distress of the attempt allows Hayt to recover the buried memories of Duncan Idaho, up to and including his death at the hands of the Imperial Sardaukar. This is the culmination of the Tleilaxu plot, as masterminded by the Face Dancer Scytale—to demonstrate to Paul that the Tleilaxu can create for him an exact duplicate of Chani—but only if he abdicates his throne and relinquishes his CHOAM holdings. Paul resists the temptation, killing Scytale himself and having Hayt/Duncan kill Bijaz.

Also in Dune Messiah, hints are dropped of a growing attraction between Idaho and Paul's sister, Alia. In one scene, Duncan even kisses Alia, which infuriates her; he replies that he took no more than what was offered. When a blinded Paul abdicates and wanders into the desert to die (as is the tradition for blind Fremen), Alia asks Idaho for his continued support and affection to help her get through the times ahead.