a Social Security number is far from unique. https://www.computerworld.com/article/2552992/not-so-unique....

Identity crisis: how Social Security numbers became our insecure national ID https://www.theverge.com/2012/9/26/3384416/social-security-n...

Back in the 1990s I was trying to convince my colleagues not to use SSNs as unique IDs. I've since noted that quite a few organizations that had gravitated to SSNs as IDs had to go through expensive and chaotic migrations to real unique IDs.