Duplicates number 2 through 6 of Calvin were created when Calvin used the Duplicator to create a duplicate of himself to do all his chores and schoolwork. This backfired, however, as the duplicate made four more of himself (he would've made more, but the duplicator overheated). The duplicates apparently assigned numbers to themselves, as they all looked identical to one another. Later, Calvin made a deal saying each one only had to go to school one day of each week and "none of us will be the wiser". They each get in trouble every time they go. Calvin stops them by tricking them under the duplicator, turning it into a Transmogrifier, and transmogrifying them into worms. When Hobbes questions this, Calvin responds that he didn't want them to be unhappy. The transmogrified duplicates were never heard from again, although the incident of the duplicates has been referred to. They are the main antagonists of the title storyline for Scientific Progress Goes "Boink".
Duplicate #2
Duplicate number two was the first duplicate of Calvin. He appeared inside the duplicator, two days after the story started; his first appearance outside of the machine was the day after. After Calvin told him to do a lot of work, the Duplicate ran away to play outside (since he possessed all the qualities of the original), but before he could, Calvin's mom sent him to Calvin's room, thinking that he was the real Calvin. When he was in Calvin's room, he created more duplicates. It is unknown how this was possible, or who pushed the button that created Duplicate #3, as Duplicate #2 must have been inside the box at the time. Duplicate number two was the first one of the Duplicates to go to school.
Duplicate #3
Duplicate number three first appeared six days after the story started. He rarely had any major appearances, because, when he was with all of the other Duplicates, it was unknown which one he was. It is unknown if he went to school.
Duplicate #4
Duplicate number four first appeared with the rest of the Duplicates, seven days after the story started. Duplicate number four was the third of the Duplicates to go to school. He was also sent to the principal's office, like Duplicate number two and Duplicate number five. It is unknown why he was sent to Principal Spittle's office, but it is known that he took some chalk.
Duplicate #5
Number 5 tries to explain his absence at school the last day
Duplicate number five was the second of the Duplicates to go to school. While in school, he is asked to demonstrate a math problem given yesterday. He tries to explain to Miss Wormwood that it wasn't him in school yesterday, but Duplicate number two, but is not believed and sent to the principal's office.
Duplicate #6
Number 6 with the others.
Duplicate number six first appeared seven days after the story started, with the rest of the Duplicates. It is unknown if he went to school, or which one of the Duplicates he was, because they were usually in one group.
Trivia
On panel three, the extra duplicate is circled.
In one panel of one strip, when the Duplicates are seen running past Calvin, a seventh duplicate with a plain shirt (instead of a striped shirt) can be seen partially out of frame. This is most likely an error on Bill Watterson's part.