Santander’s lenders inadvertently paid out US$175 million (£130m) to tens of thousands of people and businesses on Christmas Day.
About 75,000 individuals and businesses receiving one-time or regular payments from 2,000 businesses with bank accounts accidentally received a second payment on December 25.
Problem, first reported by Time, has led banks to try to recover the money, which came from the lender’s own reserves.
Payments include salaries or money from suppliers.
But because the cash has already been sent to an external bank account, it will be more difficult to get it back and Santander will have to rely on the kindness of others to do so.
Time It said account holders at Barclays, HSBC, NatWest, Co-operative Bank and Virgin Money were among those affected.
Banks can get the money back, but some are said to be concerned that it has been spent and don’t want to force their customers into overdrafts.
according to Time, one bank said it would be reluctant to take the money back if it meant the account holder could be exposed to an overdraft.
Pay UK, which runs Britain’s main payment system, is in talks about how to reverse the payments and some cash has been recovered.
As part of the “banking error recovery” process, Santander spoke with competing banks and directly contacted several clients.
A spokesperson said: “We regret that due to technical issues, some payments from our corporate clients have been improperly duplicated in the recipient’s account.
“None of our clients have been impacted by their own money as a result of this. We will be working hard with many UK banks to recover duplicate transactions in the coming days.”