Suggestion: enable metadata fields to require uniqueness

Overview of the Suggestion
We would like Dataverse software to check for uniqueness of metadata fields.

What kind of user is the suggestion intended for?
API User, Curator, Depositor

What inspired this idea?
We had a few datasets where folks reused the same ORCiD für multiple authors by accident.

What existing behavior do you want changed?
As this is something that needs to be checked on the server side, it seems like an extension of the metadata schema spec is necessary in addition to a specialized validator.

Any brand new behavior do you want to add to Dataverse?
It's not really brand new, as we already do validations on fields. The new behaviour is enabling cross-field checks. For now this is only about uniqueness, but there are probably more options.

Any open or closed issues related to this suggestion?
I couldn't find anything, but that might just be me looking for the wrong keywords during the search.

Are you thinking about creating a pull request for this issue?
Yes, we will put this on our agenda for now. But obviously we'd like to hear what others have to say first.

Start by reviewing Dataverse's existing metadata schema validation and server-side field validation described in the issue. Define how a schema would express uniqueness and how cross-field checks should behave for repeated ORCiD values. Done means the API and server-side validation consistently reject duplicates when uniqueness is enabled.