Duncan Idaho is a fictional character in the Dune universe created by Frank Herbert. He is the only character to feature in all six of the Dune novels, and is arguably the main character in the Dune series. In David Lynch's film version of Dune, Idaho was played by Richard Jordan.
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 the Arrakis fief, at the order of the emperor, 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 by the Emperor's dreaded Sardaukar by a blow to the head, in doing so saving Paul Atreides and Lady Jessica.
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, by saying to him the three deadliest insults a Fremen can hear: "You wear a collar!", "You are a servant! You've sold Fremen for their water!", and "You have no immortality! None of your descendents carry your blood!" Driven into a rage, Stilgar kills Duncan for violating the sietch's neutrality, which was what Duncan had planned from the beginning. His last gasping words were, "Two deaths for the Atreides. The second for no better reason than the first."
God Emperor of Dune reintroduces Idaho as a serial ghola: Leto has ruled as a sandworm for around 3,500 years, and has continually had an Idaho ghola to serve him at all times. To Leto, Idaho represents loyalty, humanity and the spirit of the unknown (something which the God-Emperor, having perfect prescience, knows nothing of (it is also implied that Duncan is needed for the 'Golden Path' to come to fruition). 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. Idaho's old-fashioned conscience rebels, leading the Idaho ghola to attempt to kill Leto (who is invulnerable to all but water and extreme violence as a sandworm). Leto notes that only nineteen of "his" Duncans have survived long enough to die what is considered a "natural death". In just about all of these rebellions, the reborn Idaho gets killed by the God Emperor. After a certain time, the God Emperor will order another ghola of Duncan from the Tleilaxu. The man usually shows up strongly believing he has gone insane, in shock at how the Universe has changed since his "death" (as the Duncans have memories only of the first Duncan). If the new ghola survives his first interviews, he will serve the God Emperor until his almost inevitable rebellion.
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 to the point of creating physical addiction (abilities which the Bene Gesserit also possess, but, because of the danger such enslavement poses for the longterm survival of humankind, abhor the use of), and 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 bond Duncan, the Tleilaxu's plan fruits, and he becomes aware of the memories of all the other Idaho gholas. Duncan also inherits an awkward prescient vision, wherein he sees a man and a woman staring back at him. Duncan restores the memories of the Miles Teg clone, and escapes into the Scattering at the end of Chapterhouse, evading the trap set for him by the man and woman who are implied to be Tleilaxu in origin.
Duncan's character is the symbol of newness and rebirth in the Dune universe. Leto II viewed Duncan as a symbol of the unknown, the opposite of stagnation.