"He is coming home because his father is unwell," says Wall.
"But that in itself has got some challenges because I think he still has quite a lot of resentment towards Alf from when he was a kid and at the same time, Alf doesn't trust him and never really has."
The role of Duncan was played by Lewis Devaney from 1990 to 1998, followed by Brendan McKensy who took over the role until 2005.
Duncan hasn't been seen since.
Wall says: "He was such a notorious little character. For Duncan, his behaviour as a kid has given him a need to prove himself as an adult."
Viewers can expect to see Benedict Wall on screen from late-July.