[w3c/payment-request] Prevent double spending through retry (#882)

Reading the specification, I wonder how a payment handler would be able to prevent double spending when a user issues a Payment Request retry(). 

As of my understanding, a Payment Handler probbably triggers a payment after receiving a PaymentRequestEvent.
Through the retry, a second PaymentRequestEvent is triggered, which is identical to the first one. For an unaware Payment Handler this would lead to double spending.

In case of switching Payment Method/Handler through a retry, the first Payment Handler stays unaware of the retry mechanism and the second one is unaware of this being a retry.
Which would lead to double spending as well, wouldn't it?

Am I missing something in this interpretation of the specifications?
Is there a way of the PaymentHandler to detect a retry or deal with this issue in general?



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