David Bridges, Jon Brady, Paul Crichton, Michael Gash and Wayne Purser are the other players to be shown the door as Quinn looks to rebuild his squad for next season's Blue Square Premier.
"It's a footballing decision," Quinn told the club's official website. "We've got to move forward as a club and these are the decisions that have to be made.
"I decided on the squad I want to have a couple of months ago so it's not a snap decision.
"I've made it because I think we need improvement in certain areas and it's the nature of the game that some people have to move on," he commented.
"I've thanked the players for all their efforts and help and I wish them all the best for the future."
On the decision to release Duncan after nine years, Quinn said, "I know he would have been due a testimonial next season and that's something the board will decide on; that's not my department. I've made my decision purely in footballing terms.
"The same applies to David Bridges.
"I know he's been here for a few years on and off but I'm signing a young midfielder from the youth team (Michael Hyem) and we already have two or three midfielders, so I had to decide whether an experienced player like David would get regular football, which I don't think he would."
Source: http://www.confguide.com/newsArticle.do?id=6389