I think I was getting some wires crossed and/or misunderstood what geoff (parent commentor to my last comment) was saying, so my comment may be misdirected some.

But according to The Social Security FAQ page, SSNs are not recycled, so that data (especially when compounded and hashed with other data) should be able to establish a one-to-one relationship between each primary key and an SSN, thusly having SSNs appear associated with multiple primary keys is a concern.

Other comments have pointed to other explanations for why SSNs could appear to occur multiple times, but those amount to “it appeared in a different field associated with the same primary key”. I think thats the most likely explanation of things.