SSNs are reused. Someone dies and their number gets reassigned. The database could easily be keeping track of all previous assignments for any given SSN.

Remember, SSNs are designed for social security and nothing else. They got picked up as a unique ID by private interests as a hack. They were never supposed to be as widespread in use as they are. The federal government using it this way is the specific, designed use case.

[–] Schadrach@lemmy.sdf.org 1 points 7 minutes ago

SSNs are reused. Someone dies and their number gets reassigned.

Not even that. If you were born before 2014 or so and you're from somewhere relatively populous theres a pretty good chance there's more than one living human with your SSN right now. SSN were never meant to be unique, the pairing of SSN and name was meant to be unique but no one really checked for that for most of the history of the program so it really wasn't either. The combination of SSN, name and age/birthdate should actually be unique though because of how they were assigned even back in the day.

[–] captainlezbian@lemmy.world 8 points 3 hours ago

Yep, and any attempt to replace them with a purpose designed government ID has gotten conspiracy nuts to shut it down