"In a 2011 audit, [OIG] estimated SSA overpaid approximately 24,900 beneficiaries about $623.8 million. These beneficiaries would receive approximately $869.9 million in additional overpayments over their lifetimes unless SSA corrected these payment errors.... [OIG] reviewed SSA's actions to implement the three recommendations in [the] 2011 audit and found that, although SSA made efforts, work still needs to be done. Two of the three recommendations involved systematic changes needed for SSA to effectively identify beneficiaries whose benefit must be adjusted because of WEP or GPO."