A duplicate created as a result of time travel. When one object travels to another time where that same object also exists, then two of the exact same object occupy the same time. The popular theory is that one of the paradox duplicates is doomed to destruction as a result of the universe attempting to correct this violation of the reality matrix of existence. More pressing is the belief that, were the duplicates to come in contact with one another, it would violate the very basic law of physics that 'no two objects can occupy the same space at the same time', and, thus, unmake the universe.
"That's why i didn't want us to meet, Fry. We're paradox duplicates. The more we stayed away from each other, the better chance we had of staying alive. I guess I was wrong...(cough)"
-Futurama: Bender's Big Score