In a tweet elaborating on the results of its investigation, Coinbase said that Visa payments on its website glitched, because the credit card network has changed the Merchant Category Code (MCC) for digital currency purchases. The new code apparently allows credit card providers to treat cryptocurrency buys as "cash advance" in order to charge additional fees. A Coinbase staff member revealed the code change in a Reddit post in early February, shortly before a bunch of banks decided to ban digital coin purchases using credit cards. The banks cited cryptocurrency's volatile nature and the risks associated with it as the reason behind the ban.

Visa, however, denied that it's to blame for the incidents. Its official statement published by CNET reads:

"Visa has not made any systems changes that would result in the duplicate transactions that are being reported. We are also not aware of any other merchants who are experiencing this issue. We are reaching out to this merchant's acquiring financial institution to offer assistance and to ensure cardholders are protected from unauthorized transactions."

Regardless of who's at fault, Coinbase VP Dan Romero said it's "working closely with Visa to ensure affected customers are being refunded as soon as possible." The company will also contact customers who made transactions over the past few weeks to make sure they check their banks and CC statements. On Twitter, the exchange revealed that it's working with major credit card networks to create a brand new MCC for cryptocurrency purchases -- one that won't charge customers additional cash advance fees.

2/ Over the last few months, large banks and card issuers requested that card networks change the MCC for purchases of digital currency. Visa changed the MCC for digital currency purchases to a code that allows large banks and card issuers to charge consumers additional fees. — Coinbase (@coinbase) February 16, 2018

4/ If you have been affected by any erroneous charges associated with purchases of digital currency with credit and debit cards, we are encouraging customers to contact their bank or card issuer and ask about the charges. In addition, please contact Coinbase support. — Coinbase (@coinbase) February 16, 2018

Update (February 17, 2018): A Visa spokesperson has reached out and stressed that the payment system has not changed the MCC classification of digital coin purchases. Further, the spokesperson clarified that Visa does not reverse or recharge transactions. That's not part of Visa's role -- all transactions are processed by card issuers, which are financial institutions like banks.

Coinbase and its payment processing partner WorldPay have also issued an update about the situation (emphasis ours):