Try this query.. It uses the Analytic function SUM:
SELECT * FROM
(
SELECT SUM(1) OVER(PARTITION BY ctn_no) cnt, A.*
FROM table1 a
WHERE s_ind ='Y'
)
WHERE cnt > 2
Am not sure why you are identifying a record as a duplicate if the ctn_no repeats more than 2 times. FOr me it repeats more than once it is a duplicate. In this case change the las part of the query to WHERE cnt > 1