WITH a([index], [dense_rank]) AS (
    SELECT 'aaa', 	1 UNION ALL
    SELECT 'bbb', 	2 UNION ALL
    SELECT 'bbb', 	2 UNION ALL
    SELECT 'bbb', 	2 UNION ALL
    SELECT 'ccc', 	3 UNION ALL
    SELECT 'ddd', 	4 UNION ALL
    SELECT 'eee', 	5 UNION ALL
    SELECT 'eee', 	5 UNION ALL
    SELECT 'fff', 	6
)
SELECT 
    a.[index], 
    CASE ROW_NUMBER() OVER(PARTITION BY a.[index] ORDER BY a.[dense_rank] ) 
	   WHEN 1 THEN CONVERT(VARCHAR(13), a.[dense_rank]) 
	   ELSE '' 
    END AS [dense_rank]
FROM a