Rangers' Harry Forrester
Duffy, who joined Dundee on a season-long loan deal from Celtic in the summer transfer window, featured in the Scottish Premiership game against Rangers at Dens Park on Saturday afternoon.
Mark Warburton’s side won the match 2-1, thanks to goals from Forrester and Kenny Miller.
Many Dundee fans were frustrated with the final result, as there was a suggestion that Forrester should have been sent off.
The 25-year-old former Brentford and Doncaster Rovers midfielder picked up a booking for a foul on Darren O’Dea, but he escaped caution for challenges on Cammy Kerr and Duffy.
Rangers' Harry Forrester (L) scores
Duffy, who is just 22 years of age, believes that the Rangers midfielder should have sent off.
“I thought Harry Forrester should have been off the park,” Duffy told The Glasgow Evening Times. “He should have been off even before his challenge on me because he had one on Cammy Kerr.
“I thought he could have gone for Cammy’s one and he was late on me so I don’t know how he managed to stay on the pitch after that one.
“The referee didn’t say anything to me but if he’d sent him off then it would have changed the game.”
Rangers' Harry Forrester and Steven Saunders in action
Over the course of the 90 minutes at Dens Park on Saturday afternoon, hosts Dundee had 39% of the possession, took seven shots of which three were on target, and earned five corners, according to BBC Sport.
Visitors Rangers had 61% of the possession, took nine shots of which six were on target, and earned five corners, according to BBC Sport.
Rangers' Harry Forrester (2L) celebrates with Kenny Miller (L)