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The SSN is likely to appear in multiple tables, because they will reference a central table that ties it all together. This central table will likely only contain the SSN, the birth date (from what others have been saying), as well as potentially first and last name. In this table, the entries have to be unique.
But then you might have another table, like a table listing all the physical exams, which has the SSN to be able to link it to the person’s name, but ultimately just adds more information to this one person. It does not duplicate the SSN in a way that would be bad.
Cubuntu is also a thing.