Allow duplicate assets on the manifest

[JackLewis-digirati]

Resolves #392

This PR makes it so that the DLCS coordinator is modified so that assets with the same asset id are only ingested into the DLCS once

[donaldgray]

When running locally fetching asset details is failing, happening on POST/PUT and GET

"asset": {
   "@id": "https://dlcs.host/customers/7/spaces/144/images/dup_test2",
   "error": "Unable to retrieve asset details"
}

The /allImages endpoint rejects duplicates with a 400 response.

Thought, may or may not be viable - is this duplicate check the type of thing we could do in the validator? Before it gets further into the code? Or is it okay to do further dowm?

[JackLewis-digirati]

I could swap it to do the duplicate check in the validator, but it does mean interrogating the JObject within the validator as well as in code, which I think would mean duplicating the assetId retrieval, so I felt it would be more work than if we do it at the point we already know the assetId

[donaldgray]

Single comment on how de-dup works. Also - should description be changed as there are now no changes to ManifestMerger?