Earlier quoted context omitted.
Is Stallman a new William Safire or Brian Garner? I never knew this was one of his areas of expertise. Oxford lists "doxx" as an alternate but accepted version. I don't have an OED license so I dont have access to the full etymology they use. http://www.oxforddictionaries.com/definition/english/dox
The OED does not have listings for "dox", "doxing" or "doxxing". Well, one entry for "dox" that relates to orthodoxy but that does not apply here.
I don't know why people spell it with two. Maybe they think it looks cooler? But really it's hacker slang for "I accessed and published sensitive information about an individual" and is in most cases horrible.