The Ghana and Ivory Coast internationals ensured the the Violets ended their league campaign comfortably with their efforts in front of goal
Duncan broke the deadlock at the Stadio Paolo Mazza in the 30th minute with his maiden goal for the Violets since his loan signing from Sassuolo in January.
Nine minutes later, Marco D’Alessandro equalised for SPAL as both teams headed into the interval with a 1-1 draw.
Both teams were tied for the majority of the second 45 minutes until Christian Kouame, who came on as a 55th-minute substitute for Kevin Agudelo, fired Fiorentina ahead in the 89th minute.
Eric Pulgar assisted the Ivorian forward in breaking his duck for Giuseppe Iachini’s side with his first since his loan signing from Genoa in January.
A few minutes later, Pulgar wrapped up the victory for the visitors with his strike from the penalty spot in the stoppage-time.
Duncan was replaced by Christian Dalle Mura in the 95th minute, while Burkina Faso’s Bryan Dabo, Algeria‘s Mohamed Fares and Senegal goalkeeper Demba Thiam could not stop SPAL from dropping to the Serie B.
They finished the campaign at the bottom of the Italian top flight table with 20 points from 38 matches, while Fiorentina secured the 10th spot with 49 points after the same number of games.
A ruptured cruciate ligament injury picked up during the Africa U-23 Cup of Nations in Egypt back in November delayed Kouame’s debut for the Violets until July.
He made his long-awaited return to action in the goalless draw against Cagliari on July 8 and went on to play six more games.
It remains to be seen if Fiorentina will activate the option to sign Kouame permanently in the coming days. Meanwhile, they have the obligation to sign Duncan on a permanent deal after signing him temporarily earlier this year.