Transport for London (TfL) has handed over £11,000 in refunds to around 1,800 bus commuters for charging twice the fare, according to figures released following a Freedom of Information request.
In 2012, TfL launched a new service, where commuters were allowed to pay for their bus journeys using contactless bank cards.
If a commuter had both Oyster card and contactless card in the same wallet and the commuter had already paid for a season ticket on Oyster card, then the Oyster readers detected the bank card and took money from it, charging the passenger twice.
TfL director Shashi Verma was quoted by Metro as saying that if a wallet or purse contains several cards, it is possible the reader will detect one card without seeing the other.
"Just under one per cent of all Oyster and contactless journeys involve a card clash and we are seeing this number continue to drop each week," Verma added.
TfL has already raised awareness among commuters by using posters, e-mails and public-address announcements about the possibilities of readers detecting both Oyster and bank cards, and thus reminding commuters to keep cards separately.