Count duplicate keys in table

SELECT name, email, COUNT(*)
FROM contacts
GROUP BY name, email
HAVING COUNT(*) > 1

Delete duplicates while keeping the row with lowest id

DELETE c1 FROM contacts c1
INNER JOIN contacts c2
WHERE   c1.id > c2.id
AND c1.email = c2.email;

Delete duplicates while keeping the row with highest id

DELETE c1 FROM contacts c1
INNER JOIN contacts c2
WHERE   c1.id < c2.id
AND c1.email = c2.email;