create FHIR resources and use conditional create (If-None-Exist) to avoid duplicates

Steps

1. To create unconditionally, POST the resource JSON to {base}/{ResourceType} and capture the server-assigned id from the Location header or the returned resource.
2. For a conditional create, add the header If-None-Exist with a search query string (e.g. identifier=system|value); the server creates the resource only if no match is found, otherwise returns the existing resource with 200.
3. For a conditional update (upsert), send a PUT to {base}/{ResourceType}?identifier=system|value; the server creates if absent or updates if exactly one match exists.
4. Include a resource.identifier with a system URI and value that is stable and externally meaningful so future conditional operations can find the same record.
5. Check the HTTP response code: 201 means created, 200 means matched and returned (or updated), 412 means multiple matches were found and the operation was rejected.
6. Wrap multiple related creates in a FHIR transaction Bundle (Bundle.type=transaction) so they succeed or fail atomically.