[matsemann]

The lock would normally make the second request wait, aka not return a response, until the first one is done. Then it sees it's a duplicate and returns that. Or it times out and returns an error. Then the client hopefully have some exponential back off strategy, so the third attempt doesn't suffer the same fate.

[MattDaEskimo]

If a transaction is locked then subsequent requests would return a 409, ideally with an error message indicating that it's currently being processed.