By definition we have to serialize something somewhere so we can decide which request is the success and which request is the duplicate. There's nothing special about the case of retries, this is standard concurrent programming 101. Two conflicting requests come in, which one wins?
You absolutely must wait for one request to finish before any other request can return a 409. 409 is a signal to the client that they can stop retrying, the job is done. If some request returns 409 early and the "original" request fails, you will not get further retries and the message will be lost.
Stripe's approach requires serialization as well. Only one request can succeed. If you send multiple conflicting requests in simultaneously, some of those have to block.
The good news is that we have been solving this problem for decades and we have incredibly well refined tools - database transactions and isolation levels - for solving this problem.
In my opinion, the idea of idempotency is to accept both requests, but only one is actioned (and the requester is non-the-wiser about which). Otherwise, you're just recreating database transactions - something that doesn't need to be named idempotency.
And you haven't considered multiple servers in your scenario - what if two requests meant to be idempotent with each other arrived at different servers?
How would the requester get notified if it doesn’t know which request succeeded? Is it listening for events?
And at the sake of repeating the above commenter, you solve the multiple server by serializing somewhere, because you ultimately need a lock on something. You can also perform the operation in both places and then reconcile the state later but that’s a lot more complex.
The requester doesn't want to know which request succeeded, because they are duplicates and one is a retry!
When you are using TCP, and you send the same data twice because of a delayed ack, you likewise don't care if the ACK is for the first time or the second time you sent the data. You just know the other side got the data, and that's all you care about.
You absolutely must wait for one request to finish before any other request can return a 409. 409 is a signal to the client that they can stop retrying, the job is done. If some request returns 409 early and the "original" request fails, you will not get further retries and the message will be lost.
Stripe's approach requires serialization as well. Only one request can succeed. If you send multiple conflicting requests in simultaneously, some of those have to block.
The good news is that we have been solving this problem for decades and we have incredibly well refined tools - database transactions and isolation levels - for solving this problem.