Googler opinions are my own. I work on payments stuff.

Payments at Google defined some specs for payment companies to build to, which allows easy onboarding as a form of payment on google's platform. We have a page talking about idempotency and expected behavior.

https://developers.google.com/standard-payments/reference/id...

I don't think it's that different from other payment companies, like adyen or worldpay, but we attempted to explain our view of idempotency on a single page.