'''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.
[[File:Mistake.gif|thumb|right|300px|On panel three, the extra duplicate is circled.]]
In one strip, when the Duplicates are seen running past Calvin, a '''seventh duplicate''' with a plain shirt is seen. This is probably an error on [[Bill Watterson]]'s part, but there are explanations within the logic of Calvin's imagination: the extra may be an unknown result of the duplicator burning out - he could have been an "incomplete" duplicate of Calvin. It could also be implied that after the Duplicator cooled down, it cranked out one more duplicate, but having been previously overheated, the machine had only a partial memory of what it was previously told to copy, and thus made a duplicate of Calvin without any stripes on his shirt. This theory explains the duplicate's late appearance.
[[Category:Minor characters in story arcs]]