Chris Wilder's side enters today's game against Hull City third in the Championship table and level on points with leaders Leeds.
Tracing the journey many of his team mates have taken to Bramall Lane, Duffy, previously of Prescot Cables and Vauxhall Motors, said: "It is a strength of this group that we have no superstars. There are no 'Big Time Charlies' here. Everyone just gets on with each other and works really hard.Â
"That is credit to the staff. They do the homework on the background of who comes in. We all just know our jobs.
"We love to get after teams. We enjoy playing against these so called bigger teams and these bigger players. We enjoy pitting our wits against them.
"Looking at someone who has played 300 times in the Premier League, we think, '˜Let's see what he is like under a bit of pressure'. That is our mentality."
Despite their impressive sequence of results - Wednesday's victory over Blackburn Rovers was their third straight win - United's climb up the rankings has gone largely unnoticed by opposition supporters and commentators alike.
"Not having any big household names is probably why we do go under the radar," Duffy said. "But we have a really good bunch.
"We are not bothered. Other teams get the publicity. Leeds have started fantastic, West Brom have come down and have all the Premier League players. Everyone likes to go on about their players but we just get on with our job. We are happy to go under the radar, just keep ticking the wins off."
"We are not a group who need praise," he continued. "We know how good we are and we know our jobs inside out.Â
"It doesn't matter if people on Sky Sports or Quest don't give us the plaudits. We get the plaudits from teams changing formation against us."
The visit of City could see Paul Coutts make his first appearance on home soil since the corresponding fixture last season. Six days after that match, which United won 4-1, the midfielder broke a leg during a game against Burton Albion but returned, following an 11 month absence, at Ewood Park in midweek.
"I never like to come off but when I looked over and saw it was him coming on, I was really pleased for him," Duffy, who made way for Coutts, said. "He has been such a big part of the past two-and-a-half years.
"Paul Coutts is very important to this group. Last year, there is no hiding we missing him massively.Â
"Me, him and Flecky (John Fleck) had a great understanding. It was hard to replace that. We probably didn't manage that until OIlie Norwood came in.
"He is very, very similar to Paul. It is good for the squad that we have someone capable of reaching Paul's heights.
"Paul came on the other day and did really well. He adds to the quality we have. To have him and Ollie on the pitch at the same time, all the best to the opposition trying to take the ball off them."