Find gaps or duplicates in number sequences in MySql

SELECT l.id + 1 AS START
    FROM SEQUENCE AS l 
    LEFT OUTER JOIN SEQUENCE AS r ON l.id + 1 = r.id
    WHERE r.id IS NULL

SELECT MIN(l.id + 1) AS START 
    FROM SEQUENCE AS l 
    LEFT OUTER JOIN SEQUENCE AS r ON l.id + 1 = r.id
    WHERE r.id IS NULL

SELECT START, stop FROM (
    SELECT m.id + 1 AS START, (SELECT MIN(id) - 1 FROM SEQUENCE AS x WHERE x.id > m.id) AS stop
        FROM SEQUENCE AS m
        LEFT OUTER JOIN SEQUENCE AS r ON m.id = r.id - 1
        WHERE r.id IS NULL
    ) AS x
    WHERE stop IS NOT NULL;

SELECT id, COUNT(*) FROM 
    SEQUENCE 
    GROUP BY id HAVING COUNT(*) > 1;