Duff deflected a 38th-minute shot from stand-in skipper Gareth Barry past his own goalkeeper Steve Harper to seal their fate.
It meant Magpies legend Alan Shearer had failed to work the miracle he was brought in to try and achieve with eight games remaining.
Newcastle have paid the price for a season of instability ever since Kevin Keegan decided to quit as manager in early September and Mike Ashley decided to put the club up for sale.
Joe Kinnear was brought in to try and rescue the situation until his heart problems terminated his involvement and led to Shearer getting the call.

But it was always going to be an uphill task to reverse a season of under-achievement and turmoil in such a short space of time.
Now Shearer has to decide whether he wants to take on the task of trying to help the Magpies regain their top flight status and is due to have talks with Ashley early next week.
He clearly has a massive rebuilding job ahead of them with the future of 15 players, reportedly on £50,000 or more a week, to be sorted out ahead of the less glamorous life of trips to Peterborough and Scunthorpe in the Coca-Cola Championship.
For Villa they can look back on another season of progression despite a late fade-out after automatically qualifying for Europe for the first time in 11 years.

But Martin O'Neill will need to add further quality and quantity to his squad in the summer if he is to seriously challenge for a Champions League place.
Before the kick-off Villa fans gave an emotional send-off to skipper and central defender Martin Laursen, who has been forced to retire because of a knee problem.
Gabriel Agbonlahor came close to giving Villa the lead after only two minutes when he powered a header over the bar from close range following a left-wing centre by Barry.
Villa dominated the early stages and knocked the ball around in confident style - but Brad Friedel was called upon to make the first meaningful save when he deflected a low drive from Duff around the post after he appeared to be initially unsighted.
Villa defender Carlos Cuellar then came to his side's rescue when he blocked a shot on the turn from Peter Lovenkrands on the line.

Obafemi Martins volleyed powerfully over the bar from 12 yards out after good play by Mark Viduka, before Harper reacted quickly to turn over a powerful drive from Craig Gardner.
Newcastle defender Steven Taylor became the first player to be yellow carded after 34 minutes for bringing down Agbonlahor 30 yards out.
Milner's free-kick went straight through the defensive wall but Harper was alert to the situation and able to save at the foot of a post.
The decisive goal came after Nicky Butt conceded a corner, appearing to indicate Harper had not given him a call.
The danger appeared to have been averted when Taylor headed out Ashley Young's centre - but Stiliyan Petrov headed the ball back to Barry, whose fierce shot struck Duff and completely wrong-footed Harper before finding the corner of the net.

Agbonlahor nearly doubled Villa's lead when his flick from John Carew's centre flew just wide of the far post - but Martins almost levelled in injury-time when he headed wide from Butt's free-kick.
Villa began the second half on the attack and Harper was forced to fingertip away an inswinging and powerfully driven corner from Young at full stretch.
Young took advantage of the Newcastle defence backing off to line up a shot from 20 yards out but he curled his effort past the post.
Harper was relieved when Milner dragged his low shot wide when unmarked 20 yards out as the Newcastle defence were prised open with ease.
Newcastle fans chanted "attack, attack, attack" but there was little evidence of their side threatening the Villa goal.

Shearer brought on leading scorer Michael Owen - who has been struggling with a groin injury - in place of Kevin Nolan after 66 minutes.
But Carew should have sealed Newcastle's fate when he miskicked with the goal at his mercy from an Agbonlahor centre.
David Edgar was sent off in injury-time for bringing down Ashley Young - his second bookable offence - and soon after the Magpies' fate was sealed.
This article has 11 comments
so so sorry for you Alan .
But don't get down your kneels.
I hope you and your team will be up to Barclay in 2010-2011.
I think you can.
OK?
May god bless you always.
I am from Myanmar.
By hannzaw. Posted May 30 2009 at 4:09 PM.
i felt deep pain to bite the truth...i love this club n wish every comeback
By jamie. Posted May 26 2009 at 6:24 PM.
i am so glad boro and newcastle went down they are championship teams trying for the premier league geordies bring in ant and dec
By steve jones. Posted May 25 2009 at 7:19 PM.
I am utterly delighted with the fact this team are in the championship. Now i dont have to listen to them harping on about being a big club and 'deserving' premier league football every bloody week! Reality check for them.
By Paul Cullen. Posted May 25 2009 at 11:55 AM.
i am so desparat tha last match when Daff scored own goAL I Always sympathy i tear my heat
when Newcastle go down to the coca cola champion any way this is one part of foot ball
i am compassion for Alan Sherar i hope he will back next season beacuse he an Icon
By dereje dagne. Posted May 25 2009 at 9:30 AM.
GOODBYE NEWCASTLE
You will be up again next year if you get a manager that is well proven. Make Shearer Assistant Manager and make sure he knows about all the transfers and stop buying crap.
YOU NEED TO SELL:
Beye Coloccini Cacapa Jose Enrique Geremi Barton Smith Xisco Owen Ameobi.
You wont get much but build a new team start from the back get a goalkeeper 3 class defenders 6 midfielders and get some goalscorers every team needs players to score week in week out thats why yourselves Middlesborough and West Brom went down. Tell big beer belly monster Ashley to save a few pound instead off spending it on Pies, Beer and more Pies
SPLASH THE CASH
If you wont good players you need an owner that lets the manager spend money on good palyers
Buy Kameni, someone to partner Bassong a good right back and left back Joe Ledley would be good on the Wing and good Midfielders.
By Mac. Posted May 25 2009 at 10:00 AM.
come on you gordie...make aquick come back...EPL goin to miss u...keep up shearer or find stabilise coach...
By djlocco. Posted May 25 2009 at 5:39 AM.
Remember Shearer turned down more money from Man Utd to come to Newcastle because he loved the team. We have missed his leadership since he left. The players deserved to be relegated but the club will be back hopefully with shearer in charge
By david . Posted May 25 2009 at 12:53 AM.
deservs all he gets hated Spurs and never had a good wrod to say. Hope he stays down and never gets up agains. for years we have had top listen and never a good word, goon night and good bye.
By mick Nevin. Posted May 25 2009 at 12:20 AM.
Alan Shearer "The Icon"
In his entire playing career he won the premiership with Blackburn,and then leaving for more money to Newcastle where he won nothing-now He comes back to Newcastle saying his heart is there plus he needs a 100,000 pounds a game to take them into the second division-he is without a doubt a totally useless manager,a money hungry ex player who makes more in three days than the average supporter does in a year-
By peter lawrence. Posted May 24 2009 at 6:42 PM.
why Alan Shearer took the job in the first place i dont knw but i guess he wanted a challenge and got one.well i wish them the best of luck .
By frederick yankson. Posted May 24 2009 at 6:12 PM.