Thousands of people in the UK were charged twice for debit card payments.
The error occurred due to an issue with a particular card terminal run by Cardnet, a joint venture between Lloyds Bank and First Data.
Money came out of bank accounts twice but transactions only showed up once on many shop, pub and restaurant receipts.
Lloyds Banking Group said just under 5% of Cardnet machine transactions were affected on part of 29 August and all affected customers had been refunded.
However, customers are still being advised to check their statements and contact their bank if required.
Many people banking with different organisations were charged twice for spending money in shops, restaurants and businesses using Cardnet terminals as their way of taking payment.
The majority of affected transactions involved Visa debit cards.
Simon Alton, 39, discovered he had paid twice for a meal at Côte Brasserie, in Shrewsbury, Shropshire, last Wednesday.
"It's a reminder to check your statement, it's an unexpected thing to happen," he said.
"I'm a bit lax, if it wasn't for the friends I had been out with who messaged me the next day, I might not have noticed."
Mr Alton, who works in PR, said he had spent about £60, adding: "The restaurant's been very good. As far as we know, we'll get the money back."
Côte Brasserie has been approached for comment.
David Stewart, 61, lives in Edinburgh and was charged £1,060 twice when paying a hotel for his daughter's upcoming wedding.
"I found out Saturday 1 September. At first I thought it was the hotel error. I went down to the hotel but I found out they had nothing to do with it.
"I phoned Santander on Monday morning and they said I would get my money back. The main thing has been the stress of not knowing what was happening.
"I received my money this morning. I think they should look at what's happened and give compensation as a goodwill gesture."
Other customers took to social media to express their concern about duplicate payments.
One woman, known as Lisa D, said she had visited the Oblix restaurant in The Shard, London, to celebrate her husband's birthday when she was charged twice.
@OblixRestaurant i came to celebrate my husbands birthday on Wednesday and was charged twice bith at the resturant and the bar!! How can a fine establishment do this!! I want this investigated please! Has this happened to anyone else? It was Wednesday 29/08/2018 #notimpressed— Lisa D (@shullupayaface) September 3, 2018
Sara McCluskey went to Solita NQ in Manchester with friend Oriel Hardman when both were affected.
@SolitaNQ hi, me and and @orielbird went to the NQ restaurant on Wednesday and have just seen we have both been charged twice for our meal? We split our bill between two cards but have both had the amount taken out twice. Help?— Sara McCluskey (@Macca_91) September 3, 2018
Billy Hatch visited German Donner Kebab (GDK) in Sutton, Greater London, and had the same problem.
@GermanDoner_UK Hi i recently purchased some food from your sutton store on the 29/8/18 at 8pm, the food cost me £6.49, however when I've checked my bank statement I've been charged the same transaction twice at different time, is there any way i could be refunded?— Billy Hatch (@MiniHatch) September 3, 2018
Other businesses responded by saying it was a problem with the card processing data firm and not an issue with individual businesses or banks.
Brunning Host Pubs, which runs pubs in Devon, released a statement to customers on 1 September after being made aware of the problem.
** Card transactions **— Brunning Host Pubs (@BrunningFamily) September 1, 2018
Lloyds Cardnet process our card transactions. They have advised us that come customers may have been charged twice for transactions made on the 29th August 2018. They assure us that... https://t.co/omXTaxfcvt
The Nab's Head, in Preston, Lancashire, said in its statement second duplicate payments would not be taken by them but refunded as soon as possible.
**IMPORTANT**— Nabs Head (@Nabs_Head) September 3, 2018
We have been informed of an issue with our credit/debit card payment processor FDMS where by customers may have been charged twice on either the 29th or 30th August, this will show up on your... https://t.co/88wn8Hy6ZU
Cardnet handles more than 1.2 billion transactions every year, according to its website.
It claims to make it "easy for merchants to accept card transactions online, in person or by phone".
Simon Gompertz, BBC Personal Finance Correspondent
We are all using cards more to make payments.
Last year cards topped cash for the first time ever, as contactless paying took off.
So it's vital we can have confidence in the security and reliability of the technology.
It has taken Cardnet nearly a week to send out refunds for its double-charging mistake.
In that time some customers will have been perplexed or even panicky about what happened to their accounts.
If banks want us to embrace the future, they need to be much sharper at dealing with the glitches.
A Lloyds Banking Group spokesperson said: "A small number of transactions processed last Thursday by Cardnet were duplicated.
"All customers have now been automatically refunded, however customers can check their statements and get in touch with their banks if required."
In July one of the UK's money transferring services had to deal with a backlog of payments after a system failure.
Visa experienced a similar issue in June that left customers across Europe unable to make some purchases.
While in May, TSB customers continued to face issues with online banking.