SELECT *
FROM    (SELECT * FROM Customer c WHERE c.Created >= DATEADD(MONTH, -6, GETUTCDATE())) c1
JOIN    (SELECT * FROM Customer) c2 ON c1.CustomerID <> c2.CustomerID
WHERE   (c1.DOB IS NULL OR c2.DOB IS NULL OR c1.DOB = c2.DOB)
        AND DIFFERENCE(c1.FirstName, c2.FirstName) = 4
        AND DIFFERENCE(c1.LastName, c2.LastName) = 4