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.
At the time of Frank Herbert's original novel Dune, Idaho has already undergone the long training to become a Ginaz Sword Master. He has joined the service of House Atreides and become one of Duke Leto's right-hand men (with Gurney Halleck and Thufir Hawat). When the Atreides take over the planet Arrakis (called Dune) 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 Fremen leader Stilgar and Leto both.
When the Emperor's dreaded Sardaukar attack Arrakis in the guise of Harkonnen troops, Idaho initially survives the assault and saves Paul Atreides and Lady Jessica. When they are cornered at a Botanical Testing Station, the latter flee once more while Idaho reaps an unheard-of tally of nineteen Sardaukar before finally dying from a head wound.
Idaho returns in Dune Messiah as a ghola, a clone made by the Tleilaxu and gifted to Paul Atreides, now Emperor. The purpose of the gift is 'psychic poison': the ghola — named Hayt — is supposed 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, secretly a Tleilaxu Master, 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: to demonstrate to Paul that the Tleilaxu can create for him an exact duplicate of his dead concubine, and lover, Chani — but only if he abdicates his throne and relinquishes his CHOAM holdings. Paul resists the temptation, and has Hayt/Duncan kill Bijaz before he can change his mind.
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.
In Children of Dune, Idaho is now Alia's husband and Mentat. She has assumed power as Holy Regent in the name of 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 could ever 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!" Duncan then allows himself to be struck down by an enraged Stilgar, who realizes afterward that Alia would be forced by political necessity to execute him and his only hope of survival is to join the Fremen rebels — which is what Duncan had planned from the beginning. Duncan's 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 II has ruled as a sandworm-human hybrid 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. Leto and his father also loved Duncan, which is another reason why the God Emperor wants a Duncan around. 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 as a sandworm is invulnerable to all but water and extreme violence). 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 is 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 novel significantly revolves around the perceptions and action of a new Duncan that has just been ordered from the Tleilaxu. Like the others before him, Duncan is appalled at what Atreides rule has become, but Leto appoints him as the head of his otherwise all-female military, the Fish Speakers. He also reveals that he has repeatedly used his Duncan gholas in his breeding program to genetically improve the Atreides line. The current Duncan is intended to mate with Siona Atreides, the end product of Leto's breeding program: she is able to "fade" from prescient sight. Duncan is extremely reluctant and protests, saying that he is not Leto's "stud." He does catch the eye of Hwi Noree, the Ixian Ambassador sent to Arrakis with the explicit purpose of wooing the Emperor. Leto forbids Duncan from having any relations with her, but he disobeys and sleeps with Hwi.
Duncan eventually joins Siona's rebels and works with her to assassinate Leto. His plan is successful, but Hwi is killed in the process. Leto reveals that he knew of their plan, and that his death is simply another step to ensuring the Golden Path. Siona, having seen the Golden Path during her trial in the desert, realizes she was bred as a key figure for humanity's survival. At the end of the novel, she schemes on how to carefully seduce Duncan and mate with him, as Leto had originally intended. None of their descendants will be able to be tracked by prescience, forever free of any one person's vision and ensuring that humanity could never be completely found and destroyed.
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 Miles Teg is able to restore the memories of Duncan Idaho to the ghola, but Duncan can feel that the Tleilaxu planted something else in his mind. When Murbella, an Honored Matre, tries to sexually bond Idaho, he entraps and enslaves her, revealing the Tleilaxu purpose: To conquer the Honored Matres by using a better version of their own sexual techniques. When Murbella tries to bond Duncan, the Tleilaxu's plan comes to fruition, and he becomes aware of the memories of all the other Idaho gholas. The two of them are confined to a no-ship on Chapterhouse. There, Idaho trains young men to go out into the universe and enslave Honored Matres. Duncan also inherits an awkward prescient vision, wherein he sees an old man and woman staring back at him. Duncan restores the memories of the Miles Teg ghola, and escapes in the no-ship, evading the trap set for him by the strange couple.
Brian Herbert and Kevin J. Anderson continued from the original series with Hunters of Dune and Sandworms of Dune, originally planned to be 'Dune 7' by Frank Herbert. Duncan continues his travels on the no-ship, now named the Ithaca by its passengers. He is accompanied by Miles Teg, Sheeana and a group of her Bene Gesserit sisters, the last Tleilaxu Master Scytale, and many others.
Duncan and the other refugees from Chapterhouse are still on the run from the Unknown Enemy that hunts them. They don't learn until near the end that the Enemy is the revived empire of thinking machines, and the old couple are actually the computer-entity Omnius and the robot Erasmus. In the final confrontation with Omnius, Paul Atreides (brought back as a ghola and with his power of prescience just-now restored), and then Norma Cenva (the Oracle of Time to the Navigators), both reveal that Duncan is the Ultimate Kwisatz Haderach; not Paul nor any other Atreides, nor their descendents. Overwhelmed by the unexpected revelations, Duncan decides to unite humanity and the thinking machines -- after Cenva permanently removes Omnius from existence -- rather than bring about the absolute destruction of intelligent technology for the second time in history.
Assuming the responsibility of mediator between the two sides, and with Erasmus' willing participation (the robot being left as the leader and controlling-force of the thinking machines), Duncan does something similar to Bene-Gesserit Sharing with the robot; possibly along with what seems to be a physical-bonding (similar to what Leto II did with the sandtrout) into something of a human-machine hybrid. Erasmus then voluntarily "dies", while remaining an Other Memory speaking to Duncan. In addition to the being the Final Kwisatz Haderach of the humans, Duncan becomes the new evermind of the thinking machines, guiding their joint future.
Duncan is portrayed in Dune and the other works as a handsome man, to whom women are easily attracted. He is a gifted soldier and general as well as gifted as an agent/ambassador. He can sometimes get drunk, and in Dune once calls Jessica a "damn Harkonnen spy"; she hurls coffee in his face. One of the Duncan gholas marries a woman named Irti, who strongly resembles Jessica, and it is revealed that Duncan has fantasized about Jessica as well.
Duncan is a Sword Master of the Ginaz and thus a very good fighter in all situations. In his final fight in Dune which ended in his original death, he kills 19 Sardaukar — a feat which all characters hold to be astonishing, given that a single Sardaukar supersoldier is the equal of a dozen standard soldiers in battle. The Sardaukar sell his body to the Tleilaxu, who grew the gholas of Duncan from the original cells. The original Duncan and all of the gholas show his rebellious streak, and Duncan is always a very opinionated, headstrong man.