JSON_OBJECT duplicate keys produce an invalid JSON document (RFC 8259 violation)

SELECT JSON_OBJECT('a',1,'a',2);

-- MariaDB: {"a": 1, "a": 2}   <-- duplicate keys, invalid per RFC 8259

-- MySQL  : {"a": 2}           <-- last value wins, valid

SELECT JSON_VALID(JSON_OBJECT('a',1,'a',2));   -- MariaDB: 1 (invalid document judged valid)

SELECT JSON_EXTRACT(JSON_OBJECT('a',1,'a',2),'$.a');  -- 1 (first value taken)

Violates RFC 8259 (object keys must be unique), producing a document strict parsers cannot consume;
JSON_VALID returns 1 for the duplicate-key document (verification distortion; dirty data can enter JSON columns);
Extraction takes the first value while MySQL semantics is "last value wins" — the same document resolves differently across the two systems (JSON_OBJECT results replicated from MySQL→MariaDB extract different values).