REST response code for resources that cannot be updated

I have a resource which cannot be updated (events). Hence I am allowing only Post & Get operations. (I haven't chosen PUT because the eventId is generated at server end and returned as part of response).

I already have a combination of three fields in the input which can be used to identify a unique resource.

Given this, if someone posts same request twice, this is what I am expecting to return

case 1: If unique fields already exists in DB and complete request is exactly same as previous one, then we just ignore this request (since the resource already exists).
case 2: if unique fields already exists in DB BUT other part of request is different, then send error response saying it's not supported.

What should be the correct response code to return in both case 1 & case 2.

Thanks, Harish

Solution

If the call is idempotent, meaning case 1, I would expect it to return a 200 OK because it did not really fail, it's the same as updating with the same values when doing a PUT.

If the call is not idempotent, meaning case 2, then I would expect a 409 Conflict even if the user should not actually retry the call. That's because by what you said, a user is actually allowed to send a request with the same identifiers, provided that all of the other parameters are also the same.

Also, but this is just my gut feeling, if a user tried different parameters to see the different kind of response code, he could determine the current status of a resource even if maybe he was not allowed to read it. So, depending on the scope of your application, you may want to give the same response back.