The extra duplicate is an extremely minor character who appeared in a single panel of a strip during the 1/08/1990-2/01/1990 story arc. The actual strip in where the duplicate was shown was dated 1/26/1990.
It is unknown exactly how the extra duplicate came to be. It is most likely that this was an error made by Bill Watterson, but some believe it was a faulty duplicate made by the burned-out duplicator. Notice that the extra (sixth) duplicate has no stripes on his shirt, his head is also not akin to the others as his isn't visible, implying he has no head or a severely deformed one, and also not having a visible shoulder; these are clear indications of an extra duplicate.