Depending on their family and work situation, there is a chance that an ancestor is enumerated more than once in a census. The census was not necessarily always taken “on just one day,” so individuals who moved around the time of the census may have been listed by two enumerators. Individuals who were living in one household and working as domestic help in another may show up in twice–once in each household.
© Michael John Neill, “Genealogy Tip of the Day,” http://genealogytipoftheday.blogspot.com, 22 Jun 2012.