Why do swedish spell Anderson with double ss Andersson?

Answer 1

Names ending in -son, -sen, -sson, -zen, -zon/zoon, and -ssen are classified as heritable family names. The first part of the name is called a primary patronym which is the father's name plus an affix denoting relationship (e.g. Anders, Hans, Niels, Peters, etc.)

Scandinavian patronyms were generally derived from the father's given name with the addition of a suffix meaning 'son' or 'daughter'. The most common surnames in Sweden are originally patronymic. Family names ending with the suffix "sson" are the most common names in Sweden. In 1901, the Names Adoption Act was passed, which abolished the patronymic practice. From 1901, everyone had to have a family name that was passed down to the next generation.

Andersson - Son of Anders

Petersen - Son of Peter

Nielsen - Son of Niel

As for the double s: the first s is a genitive infix (or probably just a genitive ending on the fathers name as the name was likely just a description ("Peters son", i.e. "Peter's son") from the beginning) and the second s belongs to the "son" part.