

Not even that complicated - SSN are not unique, by design. The combination of SSN and name is supposed to be, but for most of the history of the program no one was actually checking for that so it really isn’t either. Until something like 2014 the first 5 digits of your SSN were basically a code for where you were born and the last 4 were just assigned in order.
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.