Ditto is a snork from the mirror world who looks exactly like Junior; however, he has a slightly different and more sophosticated sounding voice plus an opposite personality.

Ditto seemed to treat the other snorks nicely at first, but later it is revealed that he is secretly evil and wants nothing more than to make Junior's life a misery. His only appearance is in the episode In Junior's Image
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Ditto (L) trying to shake hands with Junior

NOTE: Daffney used to have a crush on him until his true self was revealed. Being Junior's anti-self and secretly evil it might prove that Junior is secretly good. This could also contribute to the Junior and Daffney relationship in the series.