The Cobblers were beaten once again as Mathias Doumbe endured a nightmare debut in the side’s biggest defeat of the season.
Town lost 3-0 at Mansfield Town with Doumbe scoring an own goal to complete a bad afternoon for himself and the visiting defence.
Northampton had several scoring chances but once again paid the price for failing to take them while making more errors in defence.
Clive Platt should have done better with a headed effort from an early Chris Hackett cross, electing to knock the ball down for Blyth when an attempt on target was a very real possibility.
The big striker then diverted an Ian Morris cross into the path of Hackett, who did not have time to react and get a shot away, the ball bobbling harmlessly through to Alan Marriott in the Mansfield goal.
Between the two brief glimpses of goal, Matt Heath was taken off with an injury, with Joe Widdowson slotting in at left-back and Lee Collins moving to the middle.
But it was Mansfield who took the lead, Ben Hutchinson curling home a right-footed free-kick after debutant defender Mathias Doumbe had been adjudged to have fouled Calvin Andrew 25 yards out.
The strike was an expert one but the position of the ball - almost dead-central in the middle of the field - will raise question marks over Duke’s goalkeeping.
He perhaps should have done better with the second, an instinctive Sam Clucas shot that beat him on the right-hand side after the Town defence had failed to clear their lines on the edge of the box.
Carter went close with a strong free-kick that Marriott managed to beat out in the closing stages of the first half.
Town rallied in the second half and forced Marriott into two more good saves, first from a Carter free-kick and then from the advancing Morris with his weaker right foot.
The fightback, though, was stopped dead in its tracks in the most bizarre of circumstances.
A long ball over the top landed for Doumbe to clear but he sliced his attempt horribly and towards his own goal, where it beat Duke after a bounce to put the home side 3-0 up and out of sight.
Credit has to go to the Cobblers for sticking to their task, and Carter - who played reasonably well - struck a post in injury time after a strong run. But this was another deserved defeat.
Cobblers: Duke, Amankwaah, Heath, Doumbe, Collins, Hackett, Harriott, Carter, Platt, Blyth
Subs: Widdowson (for Heath 11), O’Donovan (for Harriott 74)
Not used: Tozer, Hooper, Emerton, McNamara, Toney
Mansfield: Marriott, Sutton, Dempster, Tafazolli, Beevers, McGuire, Howell, Clucas, Jennings, Hutchinson, Andrew
Subs: Stevenson (for Hutchinson 54), Rhead (for Andrew 61), Speight (for Clucas 82)
Not used: Murtagh, Meikle, Daniel, Palmer
Away fans: 460