Duncan is a fine piece of Digger nonsense. I first heard it on BFBS (British Forces Broadcasting Service) radio here in Germany in the early 1980's and I believe it was a hit for a few weeks if not months. It had that certain low-brow imbecility which appeals to servicemen. At that time I did not tape the song and gradually forgot it although snatches of it never went away. When I met Steve Kelly, I asked him whether he knew it but his memory was no better than mine.
The song is an idiotic litany about people the singer would "love to have a beer with" and their qualities, good and bad. It goes on and on and on and is great stuff for ad-libbing.
It wasn't until 1994 on my trip to New Zealand that I finally got hold of the lyrics and text. The version I sing for starters can be found in G'Day 20 All Time Favourite Aussie Songs. After the first five verses I ad-lib depending on the names of the listeners.

Lyrics