MySQL списък с всички дубликати

SELECT  a.*, b.totalCount AS Duplicate
FROM    tablename a
        INNER JOIN
        (
            SELECT  email, COUNT(*) totalCount
            FROM    tableName
            GROUP   BY email
        ) b ON a.email = b.email
WHERE   b.totalCount >= 2

• SQLPiddle Демо

за по-добра производителност добавете INDEX в колона EMail .

ИЛИ

SELECT  a.*, b.totalCount AS Duplicate
FROM    tablename a
        INNER JOIN
        (
            SELECT  email, COUNT(*) totalCount
            FROM    tableName
            GROUP   BY email
            HAVING  COUNT(*) >= 2
        ) b ON a.email = b.email

• SQLPiddle Демо