The Department of Treasury is working to return funds to more than 400,000 seniors who were double-charged by Medicare due to a billing error. 

The “process error” caused 411,000 Medicare beneficiaries who have their Part B premiums automatically deducted from their bank accounts through the “Easy Pay” system to be double-billed for the month of September, the San Francisco Chronicle reported Tuesday. 

The Treasury Department is working to reverse the double charge and about 20% of the duplicate transactions have already been returned, the Centers for Medicare & Medicaid Services stated in the report. 

The agencies are also working to notify financial institutions of the error.