When SSA becomes aware of one individual having multiple SSNs, the records are supposed to be cross-referenced in the agency's Numident database to prevent payment mistakes.
Social Security's Office of Inspector General (OIG) has done a study on how the agency is dealing with these multiple SSN cases. Here are some excerpts (footnotes omitted):
... We identified more than 5,000 instances where, contrary to Agency policy, SSA appeared to simultaneously issue monthly OASDI [Old Age, Survivors and Disability Insurance] benefits and/or SSI [Supplemental Security Income] payments to individuals under both of their cross-referred SSNs. SSA systems did not automatically identify and consider payments issued under one SSN when determining eligibility for, or computing the benefit amounts paid, under the cross-referred SSN.
Our review of 534 instances ... indicated that, in most cases, it appeared the numberholders simultaneously receiving payments under multiple cross-referred SSNs were not the same individuals, and SSA had cross-referred the SSNs in error. However, in about 20 percent of the cases, it appeared SSA had issued both payments to the same individual. ...
We also identified over 13,000 instances where SSA continued paying beneficiaries after it recorded death information on their cross-referred SSNs. ... [M]ost of the individuals receiving the payments were not the same individuals whose death information appeared on the cross-referred SSNs, and SSA had cross-referred the SSNs in error. However, in 6 percent of the cases, it appeared that SSA had assigned both cross-referred SSNs to the same individual. Based on our sample results, we estimate SSA issued about $115.4 million in improper payments to 930 deceased beneficiaries after it input death information on their cross -referred SSNs. If the Agency does not correct these errors, we estimate it will issue $13.7 million in additional improper payments over the next 12 months.
Finally, we identified approximately 2. 7 million additional instances not involving SSA payments where SSA input death information on a numberholder’s Numident record for 1 SSN but did not input the death information on the numberholder’s cross-referred SSN(s). ... [A]pproximately 97 percent of these cross- referred SSNs belonged to the same individuals. Therefore, we estimate SSA had death information for more than 2 million deceased numberholders that it did not input into the Numident. As a result, SSA erroneously excluded more than 2 million deceased numberholders’ SSNs from the DMF. ...This is one of those things where you could say that millions of dollars have been wasted. I'd say it's one of those things that shows that administering benefit payments to 67 million people is far more complicated than most people would think. Social Security will do some things to try to address this problem but there will always be problems to find because Social Security is just so big.