The pair have signed on a season-long loan.
"I am very happy to be able to wear this shirt again and reunite with my friends with whom we won promotion," wrote 20-year-old Duncan on Twitter.
"This is a wonderful place and I settled in immediately. I am ready to fight in order to secure our safety in Serie A.
"Ibrahima is my great friend and I already spoke highly of him to the club in the past. I can't wait to train and get going. I've already spoken to Coach Davide Nicola and my teammates."
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