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'''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 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.