— Duane Dibbley ("Back to Reality")
Duane is introduced in "Back to Reality", Series 5 Episode 6. The Red Dwarf came across a Despair Squid, a sea-creature, and the crew were affected by its venom which caused hallucinations in which the victim's worst nightmares come true. Cat hallucinated that he had lost his cool and style, thus creating his alter-ego Duane Dibbley.
Described by Ace Rimmer as looking so geeky he couldn't even get into a science-fiction convention, Duane displays many geek stereotypes, such as having a thermos flask, toothbrush, triple thick condom and extra strength spot cream. He has a large overbite and a pudding basin haircut, and dresses in unstylish clothing. Cat calls this version of himself the 'Duke of Dork' and 'Prince of Dorkness', describing him as "a no-style gimbo with teeth the druids could use as a place of worship". With all his illusions of style and charisma ripped, Duane was prepared to commit suicide alongside the rest of the crew. Luckily Holly had Kryten release Lithium Carbonate into Starbug's cabin, waking the crew before they could commit suicide.
In "Flashback" Cat believes once again that he is Duane Dibbley. In the hallucination, after Duane and his friends shot themselves with a single bullet, the bullet never escaped from Billy Doyle's skull, leaving Duane the only survivor. Duane was blamed for the murder of his three friends, and must now undergo medical tests for his sanity. Meanwhile, back on Red Dwarf, Holly informs the crew that Cat is hallucinating. Kryten prepares the Lithium Carbonate and sprays it on Cat, bringing him back to sanity. When Cat comes around, he finds that he has been threatening to slice his own throat with a herring.
Duane appears again in "Emohawk", when Cat's sense of cool is stolen by an Emohawk and he is horrified that he has once again become Duane.
In "Back in the Red, Part 2", Cat revives Duane once more, this time by donning false teeth and a mop head, to assist his crewmates' escape from Red Dwarf. On this occasion Lister, Kochanski, Kryten and even two skutters also disguise themselves as the Dibbleys. Later, Duane has a "dance-off" with Blue Midget, which he finally wins when he changes back to Cat. However it's ultimately revealed that this all took place within a computer simulation.
It is possible that Duane Dibbley exists in another universe. According to the dimension theory outlined in "Back to Earth, Part Three", Duane's universe would have been created as a result of Cat's hallucination.
Duane also appears on a special edition of Can't Cook, Won't Cook, titled Can't Smeg, Won't Smeg, in honour of the tenth anniversary of Red Dwarf. In this show it is revealed that Dibbley is a culinary genius.