Supermarket giant Tesco is investigating technical issues that have caused some Monzo users to be charged twice when using their card in Tesco stores.
The problem, which has been known about for several months, escalated recently when a sign was put up in a Chelmsford branch of Tesco saying that Monzo card payments would not be accepted due to technical issues.
As a result, a number of Monzo users have taken to Twitter and the challenger bank's own community forum to question what's going on.
Here, Which? explains what to do if you're charged twice for a transaction, and looks into what's behind the 'double charge' problem.
There have been some reports of shoppers being told that their contactless card transaction hasn't been recognised, leading them to try again using their Pin.
The double charge shows on the Monzo app within minutes: shoppers see the same transaction listed twice, with both sums showing as having been taken from their overall balance.
While the money from the second charge doesn't technically leave the account, it is set aside as a 'pending transaction', meaning you can't use the cash to buy anything else until the transaction is cancelled - which can take up to eight days.
Monzo began life as a prepaid card rather than a fully functioning bank account, and many people still use their accounts in this way.
This means that, rather than having gone 'full Monzo' (ie using it as their primary account), some users are only storing small sums of money with Monzo.
For those customers, even a relatively small double transaction could make the difference between having a high enough balance to make other purchases throughout the day, or being left unable to pay.
Neither Tesco nor Monzo would give us an exact figure, both saying that 'only a small number of people' have been affected by the technical issue.
When we conducted a quick poll on Twitter to ask whether people had been double-charged when using any debit card in Tesco, around one in six people said they had been. The odds were roughly the same within the Which? Money team.
This is, of course, is a very small sample size and doesn't represent the whole picture.
A Tesco spokesperson said: 'We have been made aware there could be a potential technical issue around use of Monzo cards in a very small number of stores, around duplicate charges.
'While this is not widespread, we would like to apologise to any customers affected and let them know we are working with Monzo and stores to investigate and resolve this.
'Just to be clear, customers can still use their Monzo cards in store.'
When we asked Monzo for its take on what was going on, a spokesperson told us: 'Tesco has not banned Monzo cards.
'There is a Tesco technical issue affecting a very small number of Monzo customers, where duplicate charges show in their feed; this does not debit their account twice.'
There have been reports of Mastercard - Monzo's payment provider - also being involved in the investigation. Mastercard told Which? it was 'supporting Monzo and Tesco in their investigations'.
While the double-charge problem appears to be most common in branches of Tesco, some people have reportedly experienced the same issue in Sainsbury's and on board Norwegian Air and easyJet flights.
Smaller businesses can also have problems. Kitty Cafe in Birmingham has a sign by its till that says: 'Unfortunately at this time we are unable to accept Monzo cards. We are sorry for any inconvenience caused.'
A spokesperson for the cafe told Which? that Monzo cards do not register with its tills, adding: 'We chase our till provider weekly and we are working with Monzo to try to overcome this issue. However, there doesn't seem to be a solution forthcoming any time soon.'
Hopefully, the steps that Tesco and Monzo are taking will mean that double charging becomes a thing of the past.
Until then, the good news is that you shouldn't have to do anything to get a refund for the duplicate transaction, as it should be cancelled automatically.
Tesco has advised people to call its customer service line if the money has not been returned within 48 hours (0800 50 5555 from a landline or 0330 123 4055 from a mobile), whereas Monzo told us that this kind of payment will 'automatically fix itself after seven days, or customers can get in touch with us to fix it immediately'.
Monzo has an in-app function where you can 'question this payment' - this is for mistaken transactions where money shouldn't have been taken from your account.
When one of the Which? Money team was charged twice in a Tesco store, they used the 'question this payment' function. They were advised to wait for an automatic refund, which they received after eight days.
If the double charge isn't refunded, contact Monzo via the app to let it know.