"It's a very big game for us and we know we've never been so far for so long," he said last week after his side had caused a significant upset by winning the fourth-round tie at Bolton, runaway leaders of the First Division. "We'll have to try to get a videotape of them because there won't be a chance to watch Forest before the tie. But Stuart Pearce has done a wonderful job there since he took over."
Duncan also emphasised the importance of the league to his side. This may be essential talk from those enjoying brief flirtations with Cup glory but Chesterfield have been on the fringes of a play-off position in the Second Division all season. The manager is anxious that they do not lose touch by being diverted now. "I don't like to speculate on how far this side can go," he said.
"But I do know that they have work-rate, tenacity, team spirit. It's always a difficult business. We know they can play, we help them to play together. It would be very disappointing if several of them didn't go on to achieve at a higher level with us or somebody else."
If so, some of this will be down to Duncan's careful stewardship, nurturing youth, spotting bargains, preferably free transfers from elsewhere, all the while building a team. Chesterfield have improved in each season since he took over - for the second time - exactly four years ago. After finishing 12th in the Third Division they moved to eighth, to promotion via the play-offs and to seventh in the Second Division last season.
This is unquestionable success yet Duncan, 48, might have been lost to the game completely. After a playing career whose highlight was 51 goals in 103 league matches for Spurs he went into management and had spells at Scunthorpe and Hartlepool. Moving on to Chesterfield for the first time he took them to promotion from the old Fourth Division and was subsequently offered the Ipswich manager's job.
Three seasons of "so near but so far" league positions led to the sack and Duncan became a geography teacher, using the qualifications he had gained while playing for Dundee 20 years earlier. He kept in touch with football by doing regular radio commentaries and acting as an observer for the then England manager Graham Taylor, a task which entailed compiling reports on potential international players. He was fulfilled and content but the lure of the game in which he had spent his entire adult life was too much.
"When I look back on my time as teacher I think it gave me a fresh perspective on man management. Looking after a group of kids maybe enabled me to learn ways of approaching it differently." What it taught him most, of course, is that his time in it could end tomorrow.
"Life was a lot more relaxed as a teacher but over the last three games when it's all come together and then the other night when the finishing was there too, well then it all becomes worthwhile."Reuse content