As a freshman at Classical High School in Providence, Duke stood 5 feet 6 inches (1.68 m) and played at the junior varsity level.

[2] After the season, Duke transferred to Cushing Academy in Ashburnham, Massachusetts and repeated his junior year due to reclassification.

He played with Wabissa Bede and helped his team win the New England Preparatory School Athletic Council (NEPSAC) Class AA title.

[3] As a senior, Duke averaged 17 points, five rebounds and four assists per game and helped Cushing reach the NEPSAC Class AA semifinals.

[2] A four-star recruit, he committed to playing college basketball for Providence over offers from Virginia Tech, Florida, Indiana and Villanova.

[1][4] Entering his first season at Providence, Duke shared preseason Big East Freshman of the Year honors with Jahvon Quinerly.

He helped his team win the bronze medal and recorded 16 points, four assists and three steals in a 114–75 loss to Argentina in the semifinals.