If you must use POST with a idempotency-key, then my suggestion would be to use it like you'd use a PUT: you generate a guaranteed unique URI on the client, according to a spec agreed upon with the server, along with some idempotency-key value (UUID or whatever per the recommendation of the RFC), and then POST the request to the generated URI. If you get back 200 or 201, great! If you get an error that indicates it might not have worked or you get nothing because of a partition, send the request again. If the server had already processed the first request, the second one should 400 or 409 or something, regardless of any differences between the first and second request. If there was some sort of partial processing, then some other permanent error for that particular ID or URI should convey that.

My original point, though, was that these semantics are well-understood for PUT, so just use PUT, or use POST (with the idempotency-key header) exactly as you would PUT.