When Duckett departed, Northants were 140 for one, but they were eventually dismissed for just 269.
Duckett’s fellow opener Rob Newton made 78, but no other Northants batsman could get a half-century as Glamorgan eventually got the upper hand.
Kiran Carlson was the star of the show for the away side as the 18-year-old took five for 28 on his first-class debut.
Northants had lost nine wickets for 84 runs and Glamorgan started steadily in their reply, finishing the day on 23 without loss.