Sem fazer promessas, mas aqui está uma alternativa para tentar:
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
