Futurama Episodes

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.
wikipedia.org summary:
Professor Farnsworth forbids the crew to look inside a mysterious box. Leela can’t resist taking a peek, discovering the box to be a gateway to a parallel universe.
imdb.com summary:
Prof. Farnworth creates a box which contains an entire universe and sends Leela into it, where she discovers another Planet Express with a few notable differences.