The decision comes a day after the Club announced that Mitch Hancox is also leaving St. Andrew's.
Midfielder Duffy joined Blues in June 2014 after leaving Doncaster Rovers.
During his time at the Club, 30-year-old Duffy made just seven First Team appearances and netted one goal. His last Blues start was in January 2015 in the FA Cup third round victory away at Blyth Spartans.
He spent the whole of the 2015/16 campaign on loan at Burton Albion, where he made 47 appearances for the Brewers, scoring eight times.
On Saturday, he was part of the side that won automatic promotion to the Championship after Burton picked up a point with a 0-0 draw away at Doncaster Rovers to claim second spot in League One.
Having previously been on loan at Blues, striker Wes Thomas re-joined the Club on a permanent deal in June 2014.
He has spent two spells out on loan this season. The 29-year-old was with Swindon Town for the first half of the campaign, before linking up with League One side Bradford City at the end of January 2016.
The Bantams finished their league campaign in fifth place, meaning they will play Millwall over two legs in the League One play-offs.
In total, Thomas made 39 appearances for Blues in his second stint at St. Andrew's and scored eight goals.
Canadian international defender David Edgar has been at St. Andrew's since June 2014, after joining from Burnley.
The 28-year-old figured 19 times for Blues, with his last start coming in a 1-1 draw away at Leeds United in January 2015.
He spent the tail end of the 2015/16 campaign on loan with Huddersfield Town.
For the vast majority of this season he has been on loan with Sheffield United, where he made 38 appearances for the Blades.
Following a decent start to the season, United dropped out of the promotion race and finished in 11th.
Lee Novak was signed by former Blues boss Lee Clark in May 2013, after four years with Huddersfield Town.
The 27-year-old Geordie spent the whole of the 2015/16 season on loan at League One Chesterfield, who ended the campaign in 18th place.
On a personal level, it was a successful period for Novak who finished the campaign as the Spireites' leading scorer with 15 goals.
During his Blues career Novak made 69 appearances for Blues and found the back of the net 12 times.
Neal Eardley has had an injury blighted time since making the switch to Blues in May 2013 after a four-year spell with Blackpool.
The 27-year-old Welsh international full-back twice suffered cruciate ligament injuries, which has limited his playing time to just 21 appearances.
The last time he pulled on a royal blue shirt was in the FA Cup third round home defeat to Bournemouth in January 2016.
All five leave Blues with the best wishes of everybody at the Club.