How duplicate payments are created
When a payment request is received, Adyen doesn’t check the merchant reference for uniqueness. This means that a shopper can open multiple payment sessions during checkout and pay in all opened sessions. A shopper, for example, can open and pay in multiple tabs in their browser.
On your system, these sessions have most likely the same merchant reference. However, Adyen's system sees them as unique payment requests, and creates a unique PSP reference for each of the sessions, causing duplicate payments.
How to manage duplicate payments
In most cases, a duplicate payment will be refunded automatically. However, if after checking there is indeed a double charge, we recommend you to send a refund.