Speed up MySQL join to check for duplicates

Question

I'm using the following query to return all duplicate records with the same first and last name. The trick is that the contact_id, has to be in descending order.

The query returns the contacts as expected, but it is just SO SLOW! Takes about 6-8 seconds when checking around 30,000 records.

I have the contact_firstName, contact_lastName, contact_client_id, and contact_id all indexed in the database.

Any ideas what I could do to try and speed this up a bit? Thanks for your help :)

SELECT z.contact_id, z.contact_firstName, z.contact_lastName, RIGHT(z.contact_lastName,1) AS nameNum
FROM (`contacts` x) 
JOIN `contacts` z ON `x`.`contact_firstName` = `z`.`contact_firstName` 
AND x.contact_lastName = z.contact_lastName 
AND x.contact_client_id = ".$ID." 
AND z.contact_client_id = ".$ID." 
WHERE `x`.`contact_id` < `z`.`contact_id` 
GROUP BY `z`.`contact_id` 

Answer 1

SELECT z.contact_id, z.contact_firstName, z.contact_lastName
, RIGHT(z.contact_lastName,1) AS nameNum
FROM `contacts` x
JOIN `contacts` z ON (x.contact_client_id = z.contact_client_id)
WHERE `x`.`contact_id` < `z`.`contact_id` 
And x.contact_client_id = '$id'
GROUP BY `z`.`contact_id` 

Make sure you have an index on:
- contact_id.
- contact_client_id

Answer 2

Not making any promises, but here's an alternative to try:

SELECT c.contact_id, c.contact_firstName, c.contact_lastName, RIGHT(c.contact_lastName,1) AS nameNum
    FROM (SELECT contact_firstName, contact_lastName, MIN(contact_id) AS MinID
              FROM contacts
              WHERE contact_client_id = ".$ID."
              GROUP BY contact_firstName, contact_lastName
              HAVING COUNT(*) > 1) t
        INNER JOIN contacts c
            ON t.contact_firstName = c.contact_firstName
                AND t.contact_lastName = c.contact_lastName
                AND c.contact_client_id = ".$ID."
                AND t.MinID <> c.contact_id