That 3-2 away defeat leaves Duns on 40 points from 15 games, 15 ahead of Langholm, though their Dumfries and Galloway title rivals have five games in hand.
Luke Strangeways and Sean Phillips got the visitors’ goals, with Brian Mattinson scoring twice for their hosts and Aidan Wilson getting the other.
That was one of four league games played on Saturday, one apiece in the association’s A and B divisions and two in the C division.
Spittal Rovers missed a chance to go top of the C division after being held to a 6-6 draw away to third-placed Highfields United, bringing their 100% record this season to an end.
They stay second, now with 25 points from nine games, two behind Hawick Colts and one ahead of their opponents on Saturday but with two games in hand on the former and four on the latter.
Berwick Colts move up to sixth place in the C division, on 11 points from 12 games, thanks to a 1-0 win away to Gala Fairydean Rovers Amateurs.
Six cup ties were also contested on Saturday, including Langlee Amateurs continuing their defence of the Waddell Cup by knocking Lauder out of its quarter-finals.
They join Stow and Greenlaw in the semi-finals after coming from behind to claim a 7-1 home win.
On target for the current cup-holders in Melrose, after Kai Robertson had put their opponents in front, were Des Sutherland with a hat-trick and Danny Simpson with a double, plus Shaun Hardie and Lewis McEwan.
Stow won 2-1 at Newtown, thanks to two Jordan Steele goals in as many minutes early on, with Darren Blacklock replying for the hosts.
Greenlaw beat Biggar United 5-2 at home, courtesy of two goals apiece from William Smillie and Jason Mcintosh and another from Gogz Ramsay. Chris Gordon and Ross McBratney were on target for their South Lanarkshire opposition.
Northumberland’s Spittal Rovers had already qualified after seeing off Hawick Waverley 6-0 at the start of the month.
Three Forsyth Cup quarter-finals were also played, and they saw Hawick Colts, Hawick United and Tweeddale Rovers progress after beating St Boswells 1-0, Coldstream Amateurs 3-2 and Tweeddale Rovers Colts 2-0, all away, respectively.
Ross Douglas was on target for Hawick Colts; Rab McPherson, Louis Johnstone and Harrison Hughes for United; and Robbie McNaughton and Scotty Pringles for Rovers, with Stuart Weeks and Ross Allan on the scoresheet for Coldstream.
Eyemouth United Amateurs are already through to the semi-finals after notching up a 6-2 victory against Gala Fairydean Rovers Amateurs earlier this month.