Duffy's rapid opener had a huge slice of good fortune about it as his low struck shot hit a divot before taking an uncharacteristic bounce to fly over diving Tulips goalkeeper Michael Duggan.
But it was enough to secure another win for Graham Drury's side as they maintained their push for promotion in front of a cracking attendance of 669.
The Daniels were unchanged for the third successive game and got off to a dream start in the Northern Premier League Midlands Division derby as Cosmos Matwasa played in Jack Duffy whose shot took an awkward bounce which lifted it over the diving Duggan.
It was the perfect boost for the Daniels against a visiting side who have taken points off many of Stamford's promotion rivals in recent weeks.
And the Daniels looked to build on the early initiative as they peppered the Spalding goal in the early exchanges.
Duggan had to be at full stretch on several occasions to deal with some testing crosses while Jonathon Margetts fired an inviting opening wide after a cross had landed into the path of the Stamford striker.
Abdul Koroma dragged a shot wide for the Tulips while home goalkeeper Dan Haystead saved bravely at the feet of Ben Seymour-Shove from Hart's cross following his driving run forward.
Spalding's best chance of the half fell to Stefan Broccoli - who was returning after a two-match ban - but he curled his shot wide of the far post.
Matwasa fired wide for the Daniels after linking well with Harry Vince while Margetts was denied by Duggan after an erratic clearance from the Spalding goalkeeper.
Duggan then scrambled across his goal to claw away an effort from Margetts while Spalding's Nathan Stainfield headed wide at the back post from Liam Marshall's free-kick before excelling in his more familiar defensive role to superbly turn away a Tendai Chitiza cross with Jack Duffy waiting to pounce.
Stamford had another good opening early in the second half as Chitiza raced onto Margetts' through ball but lifted his shot wide of the Spalding goal.
Duggan then showed superb reactions to block a powerful strike from Matwasa as the crucial second goal eluded Stamford which allowed Spalding to come into the contest more as an attacking force as the half wore on.
The Tulips certainly created the better openings in the final 15 minutes with Seymour-Shove showing quick feet on the edge of the area before fizzing his shot over.
Seymour-Shove then came closest to equalising as he lofted a shot past Haystead only for the post to come to Stamford's rescue.
Haystead was then at full stretch to tip over another strike from Seymour-Shove before Sean Wright made a vital goalline clearance to keep Spalding out from the resultant corner.
Former Daniels striker Gregg Smith was then thwarted late on by Haystead who made another smart stop to secure his third successive clean
sheet in a victory which leaves the Daniels in fourth spot.