Peter Taylor plans to go into next season with just those two goalkeepers on the books, having released Sergio Leite from his contract last month. Youth team keeper Curtis Aspden - who has made the bench for the first team once this season - is expected to be the third choice.
Duke was signed from Burton Albion for £20,000 two years ago, and has made 6 appearances for City.
Three other players - Alton Thelwell, Robbie Stockdale and Nick Barmby - are also out of contract in the summer. Thelwell is expected to be offered a new deal if he remains in the fold for the rest of the season. Stockdale, currently on loan to Darlington, will almost certainly be released. Barmby is likely to continue his rolling contract for another year if he is to stay.
Loanees Mark Noble and Alan Rogers also come to the end of their City contracts in the summer. Noble will return to West Ham, looking to get back in the first team at Upton Park. Taylor is considering offering Rogers, who is under contract at Nottingham Forest for another year, a permanent deal with City. Around five more signings are also expected in the summer.