The Ghana midfielder, who carries an Italian passport, has just turned 18 and big things are expected within the club of him.
"I now want to improve in every way, physically and technically. For the future, we will see," said Duncan.
On coach Andrea Stramaccioni, who has just been named as Claudio Ranieri's replacement, Duncan also told FCInterNews.it: "Stramaccioni is really a great coach, with him I am really happy. With him we have grown a lot as a team and individually, but also in personality.
"And off the pitch he gives me lots of advice to improve. Yes, for me he is like a father."