Seth Johnson's corner was initially cleared, but when Gary Kelly passed the ball back out, his first-time cross gave Duberry time to compose himself beyond a static defence.
It says much about Leeds' wretched season that Duberry, who has come through a fractured cheekbone and badlybruised ribs, became their third top scorer with just his third goal of the campaign.
Only Viduka and Smith have managed more goals as Leeds have battled with off-field problems as well as the malaise that has haunted their football.
And it was Viduka who took up the baton to put Leicester, who started the night three points ahead of Leeds, in deep trouble with a second goal as Micky Adams' team were still trying to recover from the first.
The Australia striker has hit the kind of scoring vein that could just prove Leeds' salvation before he is sold on this summer, with Middlesbrough ready to complete the move that foundered in January.
Having struck in the previous two games, Viduka conjured a marvellous finish in the 13th minute that underlined how important he will be to Gray's mission to prevent Leeds dropping into Division One.
Didier Domi's long cross caused some consternation in the Leicester penalty area, and after Smith had one prod blocked, he flicked the ball on for Viduka to execute a superb and accurate overhead kick.
It was an opening spell that recalled far better days - three years ago they were up against Valencia in a Champions League semi-final - and inspired more confidence in a side that has lacked that vital quality.
Smith hooked one audacious attempt just past the angle and Viduka pounced on Richard Scimeca's slip to unleash a fierce volley that, luckily for Walker, veered away.
Not that Leicester lay down. Matteo needed a brilliant penalty-area tackle to deny James Scowcroft, and Robinson - hurt in an early collision with Les Ferdinand - had to save from Marcus Bent and Scowcroft.
There was little he could do with the scooped volley that Ferdinand landed on the roof of the net, but Leeds nevertheless took a valuable lead into the half-time dressing-room.
Leeds might have added a third to improve a woeful goal difference, but when Jermaine Pennant cut the ball back for Smith after 52 minutes, the England striker's snap shot was well parried by Walker.
Walker was forced into another fine save when Smith hit a powerful drive he fingertipped around a post.
But Leicester began to gain some attacking momentum and silenced the home fans with their two quick strikes, before Smith had the last word.
