Our system processes transaction requests when they are sent from your platform to ours. The merchant reference in the payment request is not checked for uniqueness when a payment request is received.
This means that a shopper could have opened multiple payment sessions from your checkout, for example by opening multiple tabs in the browser, and paid in all the opened sessions. On your end, these sessions most likely have the same merchant reference. However, Adyen's system treats this as unique payment requests, and we will create a unique PSP reference for each of the sessions.