Movies:
* The Black Stallion (1979): This is his most well-known role, playing the young, fearless jockey, Alec Ramsey.
* The Black Stallion Returns (1983): Duffy reprised his role as Alec Ramsey in the sequel.
* The Secret of NIMH (1982): He provided the voice of Timothy Brisby, a young mouse.
* The Last Starfighter (1984): He played the role of Louis, a friend of the protagonist Alex Rogan.
* The Boy Who Could Fly (1986): Duffy had a lead role in this film as the young boy with the ability to fly.
* The Great Mouse Detective (1986): He provided the voice of the young boy, Olivia Flaversham.
* The Princess Bride (1987): He played the role of a farmhand.
Television:
* The Waltons (1974-1981): He appeared in several episodes as a young boy named Johnny.
* The Incredible Hulk (1978-1982): He played the role of a young man in one episode.
* The Love Boat (1977-1986): He had a recurring role as "Young Michael" in several episodes.
* Fantasy Island (1978-1984): He played the role of "Little Timmy" in one episode.
* The Facts of Life (1979-1988): He had a guest appearance in one episode.
* Knight Rider (1982-1986): He played a young boy in one episode.
* CHiPs (1977-1983): He had a guest appearance in one episode.
Note: This is not an exhaustive list. There may be other, smaller projects he has been involved in, but these are some of his most notable appearances in film and television.