get duplicate "base" URLs with mySQL
Selecting all fields makes no sense - server will output one RANDOM field value from all values in the group. Use

SELECT testtable.*
FROM testtable
JOIN ( SELECT SUBSTRING_INDEX(urlTest,'/',3) AS domain,
              field2
       FROM testtable 
       GROUP BY SUBSTRING_INDEX(urlTest,'/',3), field2 
       HAVING COUNT(*) &gt; 1 ) domains
WHERE SUBSTRING_INDEX(testtable.urlTest,'/',3) = domains.domain
  AND testtable.field2 = domains.field2


fiddle