Victory saw Rovers finish in the top half of the table, and with European football beckoning after their Worthington Cup success, manager Graeme Souness could be forgiven for a smile of self-satisfaction.
Duff received Blackburn's player of the season award before kick-off and crowned a sensational season with a superbly-worked goal.
Cole scored equally good efforts, and created and missed a hatful of other chances in an eventful game to highlight his own contribution to Blackburn's season after his £7million move from Manchester United at the turn of the year.
This may have been the end of the season but there was still plenty to play for at Ewood Park - whoever won would finish 10th if Middlesbrough lost at Leeds.
Cole should have started the party with a goal in the second minute.
David Dunn had already fooled Sylvain Legwinski with one exquisite dummy when he then beat the same player to the ball and found Keith Gillespie. His cross dropped perfectly for Cole, but somehow he headed wide.
After 17 minutes, Cole fluffed what was if anything an easier chance.
Matt Jansen, so disappointed to miss out on England's World Cup squad, was the provider, earning himself space on the left and picking out his strike partner with a delicious cross at the perfect height.
From six yards out, Cole again headed wide and then Jansen caught the bug, completely missing his kick when Duff had presented him with an excellent shooting chance.
Gillespie nearly committed a howler when his back-pass was intercepted by Steve Marlet, but the Blackburn winger atoned by blocking the shot when the
French striker attempted to curl the ball from a tight angle into an empty goal.
Fulham's patient build-up play was becoming more decisive and Louis Saha was inches wide with a low drive from 25 yards.
It was third time and unlucky for Cole after a glorious charge down the left wing by Nils-Eric Johansson.
The Swede's low ball found Cole who shot first-time, on target this time but Maik Taylor saved with his legs.
Some lovely link-up play between Duff and Cole saw the Irish winger sprint down the left and then try a return cross.
Cole had made an excellent run but this time the fault was not his, as Duff's ball was two feet too high for the striker.
Seven minutes into the second half and Cole finally got onto the scoresheet with an extraordinary goal which left Taylor red-faced with embarrassment.
Taylor received a routine back-pass from Harley, but when Cole closed him down the Fulham goalkeeper attempted to take the ball past him.
Cole executed a perfect sliding tackle and then regained his feet to roll the ball into an empty net.
Five chances, one goal. Perhaps Glenn Hoddle was right all along about that ratio.
After 65 minutes, a superb move was finished with aplomb by Duff.
Cole turned Andy Melville and sent Jansen through, and although Zat Knight slid into to force him wide, he poked a pass between two defenders for Duff to fire left-footed across Taylor and low into the corner.
It was the winger's eighth goal of a richly-rewarding season, and his seventh in the Premiership.
Cole then laid a chance on a plate for Jansen, holding off Steve Finnan before sliding a pass to his striker but Taylor was off his line to block.
A lightning dart to the near-post by Cole nearly led to a third, as he made contact with Lucas Neill's pass but could not get it on target.
With 10 minutes to go it was 3-0, thanks to Cole. Tugay's perfectly-weighted through-ball sent Cole sprinting towards goal but there was still plenty to do, and he turned Melville inside out before stabbing past Taylor to make it 13 goals for Rovers since January 1.
Lewis almost surprised Kelly from distance, before Taylor made a double save, first denying Cole his hat-trick and then blocking out Yordi.