Agregue a coluna por COUNT e, em seguida, use uma cláusula HAVING para localizar valores que apareçam mais de uma vez.
SELECT column_name, COUNT(column_name)
FROM table_name
GROUP BY column_name
HAVING COUNT(column_name) > 1;
SELECT column_name, COUNT(column_name)
FROM table_name
GROUP BY column_name
HAVING COUNT(column_name) > 1;