The Ghanaian midfielder made his long-awaited first appearance in La Viola colours
He started alongside Gaetano Castrovilli and Milan Badelj in a three-man midfield, the latter getting sent off for two bookable offences in the first half.
Duncan was also booked for a foul in first-half added time.
Morten Thorsby handed the away team the lead with an own goal, followed by Dusan Vlahovic and Federico Chiesa netting braces including one each from the penalty spot.
Former Southampton striker Manolo Gabbiadini scored what was nothing but a mere consolation in the 90th minute for the home team.
— ACF Fiorentina English 🧢 (@ACFFiorentinaEN) February 16, 2020
Duncan was on until the 71st minute when he was replaced by Marco Benassi, to rousing applause from the fans.
The Ghanaian midfielder made 40 touches and 25 accurate passes (86.2%).
He had 100% success with his long balls and dribbles – both two attempts from two.
The result moves Fiorentina up to 13th on the log, seven points away from the final Europa League qualification spot.
They next play as hosts to AC Milan at the Artemio Franchi on February 22.