The first trial
The Duplicator has only been used twice. The first time it was used, Calvin made a duplicate of himself to clean his room while he went outside. The clone objected, and went to play outside. After the clone left, Hobbes remarked, "He's a duplicate of you, all right," to which Calvin replied, "What do you mean? This guy is a total jerk!"
Calvin's mother ushered the clone back inside, asking him why he wasn't cleaning his room, as the real Calvin and Hobbes went outside. Calvin's mom went outside again, and said to the real Calvin, "Didn't I just send you to clean your room?"
When Calvin returned upstairs, he found the clone had made four more duplicates. Hobbes said, "Your machine is a big success." One of the duplicates replied, "Are you kidding? It burned out after the fifth one of us!" The clones alternated days of going to school. This got the real Calvin in trouble, and he tricked them under the box, and changed it back into the Transmogrifier. He changed them into worms, and buried them outside.
The second trial
The second time, Calvin added an Ethicator to perfect his Duplicator, making a manifestation of his good side. He sent the duplicate to school, and to do chores. When he tried to explain to Susie that he was Calvin's good side, she didn't believe him, stating: "If that was true, you'd be a lot smaller."
He offered to carry Susie's books, but she said no, claiming he'd just throw them in mud. He wrote her a Valentine, but she just thought he was being sarcastic. When the real Calvin found out, he and the duplicate got in a fight. When the clone had an evil thought (he wanted to strangle Calvin), he disappeared, thanks to the Moral Compromise Spectral Release Phantasmatron. The Duplicator was never used again afterwards.