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; this was a clear indication of an extra duplicate.