The pair scored within four minutes of each other as the visitors rallied having gone behind to a Steve Elliot own goal.
The home side were given a boost early on when experienced defender Elliot deflected Beckford's strike past goalkeeper Steve Phillips, his second own goal in two seasons at Elland Road.
But Paul Trollope's men - still buoyant after their 6-1 drubbing of Hereford - regained their composure. First Lambert showcased his fine finishing by volleying low past Leeds keeper David Lucas from 12 yards - notching his fifth goal of the season.
Four minutes later Duffy gave them the lead after Stuart Campbell found enough space on the right wing to whip in an inviting cross and the poacher was in position to pounce from six yards.
But substitute Enoch Showunmi teed up Beckford to save the day by chipping the ball in for the equaliser 15 minutes from time.