[Groxx]

If the first is still in progress and a second arrives, you can wait for it to finish (and then return whatever would be useful, e.g. you could replay the first request's response), or fail the second (409, 423, 429, there are a number of codes that could apply) to tell the caller to retry later.

So yeah, you can do basically anything that isn't inconsistent. Success, fail, delay, don't respond until timeout, all are valid as long as you don't double-apply. Most concurrent systems are like this in some way, because all successes can become errors, and all responses might never arrive. It has nothing really to do with idempotency.