The Ethicator explained.
The Ethicator is an add-on device for the Duplicator which allows the user to control the moral characteristics of the duplicate. Calvin only used this add-on once, to make a duplicate of his good side to do his dirty work, including going to school.
Despite its apparent usefulness, the Ethicator was a failure, evidenced when the duplicate began writing embarassing love letters to Susie Derkins. Calvin had unintentionally integrated a Moral Compromise Spectral Release Phantasmatron into the Ethicator, meaning that if a "good" duplicate had an evil thought, it would vanish to prevent defeating his purpose. Presumably, it worked the opposite way for a "bad" duplicate.