The Farnsworth Parabox

When the Professor conducts an experiment so hideous that he wants his apparatus destroyed in the thermonuclear inferno of the sun, he has Leela guard the two-foot cubic box overnight so that no one opens it and sets it off. Fry and Bender try their best to sneak a peek but it is Leela who finally gives in and opens the box. Drawn into the power of the box, Leela is thrown into a parallel universe, which has duplicates of the Planet Express gang - including Leela.