An error caused 5% of Cardnet Merchant Services Ltd's cardholders to be charged twice, on 5% of transactions at its card terminals, Sky News reported.
The double charge caused a furor on social media. The majority of those affected were using Visa debit cards, according to the report.
Cardnet is jointly run by Lloyds Banking Group PLC's unit Lloyds Bank PLC and KKR & Co.'s unit First Data Resources.
The duplicate transactions occurred on Aug. 29. Most of the affected cardholders were refunded by Sept. 4, Cardnet said.