-- 1. 삭제 대상 확인 --
SELECT *
  FROM t a
 WHERE EXISTS (SELECT 1
                 FROM t b
                WHERE b.user_id = a.user_id
                  AND b.val     = a.val
                  AND b.idx     < a.idx
               )
;
-- 2. 삭제 --
DELETE
  FROM t a
 WHERE EXISTS (SELECT 1
                 FROM t b
                WHERE b.user_id = a.user_id
                  AND b.val     = a.val
                  AND b.idx     < a.idx
               )
;

마농님 감사합니다.

(감사 답변이 늦어 죄송합니다)