Pokud DB2 zná join (nevím, nikdy jsem v ní nepracoval), snad by mohlo pomoci :
SELECT DISTINCT d1.klcitc,d1.kl_mob
FROM klnt AS d1 JOIN klnt AS d2 ON d1.kl_mob=d2.kl_mob
WHERE d1.kl_mob <> '' and d1.klcitc>d2.klcitc
ORDER BY d1.kl_mob
pokud jsem neflákl chybu v syntaxi ( neodzkoušeno) .