Season 4 Episode 2:
Divided We Stand
Ben gains another Omnitrix hero to use, which he calls Ditto. Gwen can't stand being around the multiplying Dittos, and they end up giving her headaches. The Dittos, though, prove to be real helpful against Dr. Animo and his multiplying Stinkflies after he extracted some of Ditto's DNA.
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About Ben 10:
When 10-year-old Ben Tennyson discovers a mysterious device, he gains the power to change into ten different alien heroes, each with uniquely awesome powers. With such abilities at his disposal, Ben realizes a greater responsibility to help others and stop evildoers, but that doesn't mean he's above a little superpowered mischief now and then.