index.json: make org.opencontainers.image.ref.name unique?

While trying to implement index.json parsing in umoci I've come to a bit of a worrying realisation. There isn't really any explanation of how cases where org.opencontainers.ref.name is not unique should be handled. While I understand the case where you have multiple application/vnd.oci.image.manifest.v1+json descriptors with different platform entries (though I think that doesn't make sense in light of the fact that application/vnd.oci.image.index.v1+json has platform entries too), it doesn't really make sense to me in the general case to allow org.opencontainers.ref.names to be duplicated in index.json.

So here's two things that would be nice if we could make org.opencontainers.ref.names unique. This includes the multiple-platform case I mention above because it starts to get a bit hairy -- and honestly you should just be using application/vnd.oci.image.index.v1+json for that.

Sorry to stir this pot again, I've only just hit this issue trying to fix up the oci/cas implementation in umoci.

#533

Start by reading this issue alongside #533, then inspect the OCI index.json behavior referenced by umoci and its oci/cas implementation. Determine the intended handling of duplicate org.opencontainers.ref.name values and whether the specification needs clarification or a normative change. Done means the project reaches agreement on the rule and records it in the relevant specification text.