Duane Dibbley is one of The Cat's Alter-Egos.
We first met Duane in "Back to Reality", Series 5 Episode 6. When the Boys from the Dwarf came across a Despair Squid, a sea-creature that secretes a venom which brings on hallucinations that makes your worst nightmares come true, thus driving its victim to suicide, The Cat's hallucination caused him to lose his cool and style, thus creating his alter-ego, Duane Dibbley. Looking so geeky he couldn't even get into a Science-Fiction convention, Duane stumbles through life clutching his thermos, toothbrush (he certainly needs that one!) and extra strength spot cream. Even The Cat admits that he's the 'Duke of Dork' and also the 'Prince of Dorkness', and describes his alter ego 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 away his transformation, Duane was prepared to commit suicide alongside the rest of the crew. Luckily, Holly woke the crew up by having Kryten release Lithium Carbonate into Starbug's cabin before they commited suicide.
We met up with Duane again in "Emohawk", when Cat's most important emotions are stolen by a Polymorph. Once again, he slips into his sideways-ironed flares and nylon shirt to do battle with his foes. Or rather attempts to. And fails, miserably.
In "Back in the Red, Part 2", The Cat revives Duane once more, this time by donning false teeth and a mop head, in order to assist his crewmates' escape from Red Dwarf. On this occasion Lister, Kochanski, Kryten and even two skutters also disguise themselves as Dibbleys. Later, Duane has a "dance-off" with Blue Midget, which he finally wins when he changes back to Cat. (Ultimately, however, it's revealed that this is all within a computer simulation.)
It is also possible that Duane Dibbly exists in another universe and it is unknown to his origin in the case it could be that the Cat encountered him after the events of Series VIII of those other facts through the 9 year journey. Following the dimension theory outlined in "Back to Earth, Part Three", a version of him does exist in the universe created as a result of the crew's hallucination in "Back to Reality".
He makes a special appearance on the special edition of "Can't cook, won't cook" in honor of the 10th anniversary of Red Dwarf, which reveals that he is a culinary genius.