Lloyds Bank said that there was a "technical error" in their merchant system on the 31st of December, and that all the duplicated transactions have been corrected, according to this story at ComputerWorldUK.
However, since the error may have affected non-Lloyd bank customers as well, reversing duplicate transactions may not be the end of it. The reason is that some of those non-Lloyds Bank customers may have incurred penalties from their bank or credit/debit card holder for being "over-drawn" by the duplicated transaction. Those folks are going to have to contact Lloyds directly to get the issue straightened out.
"Cardholders who believe they have been financially penalised by their bank or card issuer as a direct result of the duplicated transactions are advised to contact us... We will consider all reasonable and evidenced requests for reimbursement."
To those so affected, I say, "Good luck!"
Oh, before I forget, Lloyds "would like to apologise to affected individuals for any inconvenience caused."
Robert N. Charette is a Contributing Editor to IEEE Spectrum and an acknowledged international authority on information technology and systems risk management. A self-described “risk ecologist,” he is interested in the intersections of business, political, technological, and societal risks. Charette is an award-winning author of multiple books and numerous articles on the subjects of risk management, project and program management, innovation, and entrepreneurship. A Life Senior Member of the IEEE, Charette was a recipient of the IEEE Computer Society’s Golden Core Award in 2008.