The 21-year-old has made quite a rise into manager Bobby Gould's first team over the last couple of months, after signing his first professional contract with the club in the summer of 2000.
Duff made his league debut this season against Division Two high-flyers Crewe Alexandra on Boxing Day and has gone on to establish himself in the Robins' back-line alongside brother Michael Duff.
His progress at club level has earned him a call-up to the Northern Ireland Under-21 squad this season.
"I'm very happy to get the new contract sorted out," Duff told the club's official website.
"For the last couple of years, nothing was sorted out until the end of the season and only then it was a one-year extension but now I have got it all sorted out before the end of the year.
"The manager has kept pushing me and encouraging me, telling me he had faith in me.
"For the first few games, I was really nervous but now I'm getting a bit more confident and I just have to keep putting in performances.
"This new contract is another step so I just have to take it on now."
