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I doubt that there is an actual standard HTTP response code to indicate exactly what you're looking for. `202` (Accepted) is close, but doesn't really seem to me to be about a situation where the client should resubmit the request; by the definition in the RFCs, it's more of a "the request has been added to the work queue" than a "no action has yet been taken, but the request is acceptable for resubmission".

Even the formal definitions of these status codes go back to [1996](https://datatracker.ietf.org/doc/html/rfc1945#section-9.2) (RFC 1945) and don't seem to have changed much [since then](https://datatracker.ietf.org/doc/html/rfc7231#section-6.3). The Web was a very different place back then...

Personally, since the server understood and was able to successfully parse and process the request (only that the outcome isn't yet finalized), I would probably opt for the good old `200` (OK) response with a machine-readable response body indicating to the client that further confirmation is required. For cases where no further confirmation is required, the response would be similar but the response body would indicate either proper success or failure. The response body could be JSON, XML, or something else, depending on other requirements.

The contract between the client and the server then becomes something like HTTP `200` for "meaningfully processed request", and the response body indicating further details about the outcome of that processing. That seems to me to be roughly in line with the intent of an "OK" response from the server, even when the request resulted in no actual action being taken.