The former Republic of Ireland international, 34, has made over 150 appearances for the Cottagers since signing from Newcastle United.
He only signed a new twelve month extension with the club back in January, keeping him at Craven Cottage until next summer, and is set to be offered another new deal providing he stays fit.
Duff began his career with Blackburn Rovers, and, after seven seasons at Ewood Park, secured a big money move to Chelsea following Roman Abramovich’s arrival in London.
He won two Premier League titles and the League Cup during his time with Chelsea, to add to the League Cup he won with Blackburn under Graeme Souness in 2002.
He was then transferred to Newcastle United in 2006, and was part of the team that were relegated to the Championship in 2009, moving to Fulham after playing just one game in the second-tier with the Magpies.