Duncan is Garp and Helen's first child, and he grows up to be one odd bird.
In many ways, he's a lot like his father. They're both artists; they both marry spouses who make them believe in their creativity; and they both share a wry, irreverent sense of humor. But there's one thing that Garp has in spades that Duncan can't quite grasp: determination.
Much attention is paid to Garp's "unnaturally mature" demeanor and "discomforting calmness" (2.27) at a young age. Duncan, on the other hand, doesn't take control of his life until a near-death experience forces him to. Although, in fairness, perhaps even here a similarity emerges between father and son: Garp, too, fails to appreciate what he has until he almost loses it. It's just that in Garp's case, this pertains to Helen instead of, say, his career.
Importantly, Helen observes at one point that Garp is keeping his son "anxious [...] tense" and "immature" (10.28). When she does, it's clear that somewhere along the way, Garp loses his unnatural maturity—otherwise he wouldn't be doing this to his son. So although Garp starts his adult life with clear goals, a long string of infidelities and personal tragedies keep him from fully realizing them. By contrast, Duncan, though less determined than his father, seems to actually become happy.
Duncan, then, is a bit of an alternate-Garp. They have a whole lot in common, but their differences cause them to have very difference experiences of their lives—and Duncan's generally seems happier.