In 2009, Duncan was surprised to see his former lawyer and drinking buddy Jeff Winger enrolled at Greendale who had been disbarred until he earned a legitimate degree. Since Duncan owed Jeff for helping him get out of a D.U.I. charge years ago he asks that Ian get him every test answer that semester. Duncan balks at the request and tries to talk Jeff out of continuing his cycle of cheating to get ahead. After Jeff continued his demands Duncan made acquiesced but only on the condition that Jeff give him his Lexus. Ian also offered his own mini coup in exchange as part of the bargain so Jeff could have a vehicle. Jeff reluctantly agreed but afterwards discovered that Duncan just gives him an envelope of blank paper. Duncan was enjoying a glass of wine when a Jeff stormed into his office grabbed his car keys furious that Ian tried to teach him a lesson ("Pilot")
Duncan set up a psych lab and invited his top students to help prove his hypothesis, "The Duncan Principle." He posited that the longer a subject was left to wait for something the more dramatic the person's subsequent breakdown would be. Annie Edison managed to join Duncan's pupils in observing the experiment and recruited her friends Troy Barnes and Abed Nadir to participate. Duncan allowed her to start the lab once all the test subjects were gathered in one room. Ian was amused when one of the guinea pigs, Ben Chang, immediately went berserk and stormed out. As the hours passed the room emptied and only Abed remained which frustrated Duncan. Furious that Abed wouldn't leave, Duncan angrily criticized Annie for bringing him. His class then started to document his breakdown which made Duncan cancel the experiment and end the test lab ("Social Psychology").
Duncan served on a faculty tribunal alongside Dean Pelton and Ben Chang to deal with Britta Perry who cheated on a Spanish exam. The trial was being held in Borchert Hall at the edge of the swimming pool. Duncan and Chang's animosity towards each other reappeared as the two continually sniped at each other. During the recess they went off to clear the air between them. They argued all the way into the shower stalls in the men's locker rooms where Chang revealed he felt continually disrespected by Duncan. Their conversation was interrupted when they realized there was an overweight naked man showering in front of them. Back at the trial, Duncan and the rest of the tribunal were swayed by Jeff's argument that Britta should not be expelled for her actions. Duncan decided that she should instead be sentenced to a year's worth of therapy sessions conducted by him ("Advanced Criminal Law").
At the end of the school year Duncan was drunk again while attending the school's transfer formal. While at the dance he insulted the Dean which led to an immediate suspension. Chang at that point had been fired due to falsified credentials and enrolled himself at Greendale to earn a legitimate teaching degree. Duncan had mocked him earlier for his situation, but Chang couldn't retaliate physically or he could be kicked out of school. Once Ian was suspended, Chang took advantage of that fact to hit him in the face with a roll of quarters ("Pascal's Triangle Revisited").