One of the best Duncan episodes. I don't think this is the episode but Duncan explains on the phone to a girl who he is- "Dunkie... with a U..." and when she still doesn't seem to get it he elaborates "Dobie... with a B." CLASSIC.
Usually Duncan doesn't work extremely well. He was added in the last season to appeal to younger viewers since Dwayne Hickman went from looking 15 to 35 over the course of the series! His primary character trait is that he has Dobie's obsessions (girls) crossed with Maynard's spaciness. Hence he is really just a hybrid of the two leads.
This can leave him a bit vapid and repetitive, although he does a good job of holding his own with Bob Denver- no mean task.
In this episode most of the other regulars are largely absent- leaving Mayard and Duncan to carry most of the episode quite ably. They come off as naive rather than stupid- which is what gives it its charm.