The 36-year-old former Leeds and Charlton forward, who also counts Derby, Sheffield Wednesday, Portsmouth, Blackburn and Ipswich among his former clubs, spent last season at Burton and has now returned to football on a contract until the end of the term after leaving Albion in the summer.
Varney impressed during a practice match while training at Whaddon Road this week and Duff believes he will bring some extra quality to the League Two side.
He told the club's official website: 'Luke is someone I know well and have known for a long time. I've played against him many times and I think he can add some real quality to the group.
'His experience will help, his professionalism with definitely help and his character will help the dressing room as well.
'He is a fit lad. People will question his age but I know, having played to older than he is, that as long as you still have that fitness then age is not an issue.
'In the game he played this week his numbers were better than anyone else in the team so he has shown that he can still run. Having played at a higher level as recently as last season we know that he also has the quality to improve the group.
'He hasn't come here for a payday - you don't go to Cheltenham Town for that - but he knows where we're at, we know where he is at and I think it will be a good fit all round.'