Steps to replicate the issue

• create an item redirect
• make a request to the redirected item that would either result in its redirect target to have the same label and description
• OR make a request to the redirected item that would cause its redirect target to have the same label and description as another item (collision)

What happens?:

• the API responds with a 400 label-description-same-value or item-label-description-duplicate

What should have happened instead?:

• the API responds with a 409 redirected-item

Technical note:

• We probably don't want WikibaseEntityLookupItemDataRetriever to resolve redirects. Making it return null in case of a redirect would likely result in the expected behavior.