Mr. Duncan (voiced by Greg) is one of the teachers that oversaw the Blue Light Disco in 'The Blue Light Disco Sketch' (see Episode 6). He describes himself as very sexual and has ‘had a gutful’ of fingerings and poops on toilet lids.
We hear in Episode 102 that Mr. Duncan has been teaching since the 80s where it was very common to find soiled underpants in bins.