2 Answers
Kevin Davidson, 30 years in Vital Statistics software systems in the USA

The primary cause of two people having the same social-security number is someone using a number that is not theirs—either identity theft or a typographical error.

Odds someone else has your SSN? One in 7

SSA states that it will issue a new number if it is assigned to more than one person (implying this can happen). My impression of what I have read on this subject is that a duplicate number assignment is more likely to happen when two people with the same name, same birth date and other identifying information apply for numbers.

I’ve worked on large statewide vital records databases and was surprised how often I saw identical names and dates of birth.

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Ron Jarrell, Technical Architect for Campus Surveillance System at Virginia Tech

It happened sometimes when people with very similar info applied at about the same time. Or sometimes someone will just be confused about what their number is and use the wrong one, because they thought they were 578000000 and it turns out they were really 579000000, and just wrote it down wrong.

There are some famously “retired” numbers that are a result of such error; there was a company that sold wallets with a place for your SS card, in the very early days of the whole concept. It came with a fake card to show where you’d put your card. An amazing number of people thought that that was their SS number (how convenient the wallet came with their SS card!) and used it, causing the SSA to have to block the number and try to unsnarl the people using it

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