On the back of three straight wins, Middlesex chose to bat first against leaders Yorkshire at Radlett, but quickly slipped to 1-2 after just seven balls.
It set the template for the innings as Ben Coad (3-26) and Matt Milnes (4-29) ran through the hosts who subsided to 129 all out in just 31 overs – the lowest-ever List A score at the Middlesex out-ground.
In reply, Adam Lyth (39) and Imam put on 59 for the first wicket before three quick wickets from pacer Henry Brookes saw Yorkshire slip to 67-3.
But it proved just a brief wobble as Duke followed up his six catches with an unbeaten 28 in an unbroken stand of 63 with Imam (54 not out).
Having started the day in second, the heavy defeat saw Middlesex end the day in fifth, with four teams separated by net run rate with three group games to go.
The group includes Somerset and Warwickshire, who played out a thriller at Taunton.
Asked to bat first, Somerset started strongly as Tom Lammonby and James Rew added 143 for the third wicket, opener Lammonby falling a ball after bringing up a 110-ball century.
Skipper Rew made a typically aggressive 81 off 69 balls before his younger brother Thomas plundered three sixes in a 29-ball 41 as the hosts posted 309-8.
Ben Green (3-65) removed Bears’ top-order batters Rob Yates (47) and Zen Malik (44), but runs came all the way down, with Hamza Shaikh adding 44 to keep the visitors in touch.
Jake Lintott fell within touching distance after an aggressive 36-ball 50, but Kai Smith (45 not out) struck the winning run with just three balls to spare.
Sussex make up the quartet after moving up to fourth after an impressive chase in Northampton.
Northamptonshire looked set to post a total well above 300 as Stuart van der Merwe hit 59 on his professional debut in a 115-run fourth-wicket stand with James Sales.
Having brought up his maiden senior ton on Friday against Durham, Sales fell two runs short of completing back-to-back centuries, as the Sharks fought back well at the death to keep the hosts to 295-8.
Sussex openers Tear and Tom Haines (76) then made the most of a lightning-fast outfield to put on 216 in 33 overs and break the back of the chase.
Haines finally went for 76, but Tear kept on going, bringing up his first List A ton and hitting 23 fours in a 146-ball knock of 159 to tee up a seven-wicket win with 19 balls to spare.
And Kent remain bottom despite chalking up their first win at home, against seventh-placed Lancashire.
BJ Dawkins impressed with a 79-ball 85 in just his second senior appearance, as he put on 155 with Jayden Denly, who fell nine runs short of a maiden List A ton.
Grant Stewart then smashed three sixes in a rapid 32 to take the home side to a challenging 315-8.
The Spitfires looked set to wrap up a comfortable early win as the spin of Matt Parkinson (3-57) reduced Lancashire to 189-6, yet Josh Bohannon made them sweat.
Coming in at 38-2, Bohannon put on 96 with Marcus Harris (54) and took the Red Rose within 30 of the target, hitting 17 fours and two sixes.
But soon after he was ninth wicket to fall for 133, off 110 balls, the visitors were bowled out for 293.
