The Ghanaian made a memorable comeback to his one-time home ground for the matchday 20 tie, having played for Inter between 2012 and 2014 after developing through the youth system.
His return, however, coincided with a sanction on the club to play behind closed doors as punishment for a recent racist attack on Napoli's Senegalese centre-back Kalidou Koulibaly during a league fixture.
“I’m happy to be back at San Siro and I hope the fans have a good time,” Duncan told Inter TV.
“The initiative from Inter against racism is very important, as it sends a message to everyone on an issue that is not good for the sport. The club is dealing with the situation well because those who insult are just a minority.
“The fact that it’s played behind closed doors changes things for us, too."
Duncan lasted for 73 minutes in Saturday's fixture which ended in a scoreless draw.
The game took his league tally to 13 games - involving 13 starts and three goals - so far this season.
According to transfer reports, Inter are interested in re-signing the 25-year-old this January.
His compatriot and club-mate Kevin-Prince Boateng, meanwhile, lasted for the entire duration of Saturday's game, same as countryman Kwadwo Asamoah, who played for Inter.