> unless there are people who share the same SSN That happens all the time. First of all people steal SSN's and use them (and you are not the police, so it's n

> unless there are people who share the same SSN That happens all the time. First of all people steal SSN's and use them (and you are not the police, so it's not your responsibility to do anything about that). Second people make up fake SSN's because they don't want to give you their SSN. People also make typo's, and you can end up with the same SSN. An SSN is not unique in the real world.