Adding "ss" to names?

Can you tell me if there is a right way or wrong way to add ss to the son's of the father? Like Guttormsson or Guttormson - I noticed in a lot of the Bygland books they just use one s. Also in Valle Kommune books they use a double ss. It is confusing on ancestry because they maybe under both names and I have been changing them to one s. Anyone on here close to Grand Forks, ND - Chester Fritz Library has a wonderful collection of books in their genealogy department.