Episode 66 of "Futurama"
s05e10 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.