When I looked him up on Wikipedia to make sure I was thinking if the same guy, I learned there was also a Robert Hansen (with one S) who was a serial killer. Kind of a cursed name, really..
It’s a very common name among Danes, Norwegians (in fact their most common surname) and Schleswig-Holstein Germans and the descendants of these groups. When I hear the name I mainly think about a Danish actor. Hanssen is a Norwegian variant (the one s is still more common) and Hansen is the Danish version.
It’s the Danish and Norwegian version of Johnson (as Hans is the most common version of John in Scandinavian and German, the other common version is Johan).