数据库优化顾问建议创建现有索引

当我运行SQL Server 2005数据库优化顾问时,它给出了创建索引的建议,但它将建议对已经有索引的列进行索引。为什么它会给出再次创建相同索引的建议?

以下是我的SQL:

SELECT t.name AS 'affected_table'
  , 'Create NonClustered Index IX_' + t.name + '_' 
   + CAST(ddmid.index_handle AS VARCHAR(10))
   + ' On ' + ddmid.STATEMENT 
   + ' (' + IsNull(ddmid.equality_columns,'') 
   + CASE 
     WHEN ddmid.equality_columns IS NOT NULL 
          AND ddmid.inequality_columns IS NOT NULL
     THEN ',' 
     ELSE '' 
     END 
   + ISNULL(ddmid.inequality_columns, '')
   + ')' 
   + ISNULL(' Include (' + ddmid.included_columns + ');', ';') 
  AS sql_statement
  , ddmigs.user_seeks
  , ddmigs.user_scans
  , CAST((ddmigs.user_seeks + ddmigs.user_scans) 
    * ddmigs.avg_user_impact AS INT) AS 'est_impact'
  , ddmigs.last_user_seek
FROM 
  sys.dm_db_missing_index_groups AS ddmig
  INNER JOIN sys.dm_db_missing_index_group_stats AS ddmigs
    ON ddmigs.group_handle = ddmig.index_group_handle
  INNER JOIN sys.dm_db_missing_index_details AS ddmid 
    ON ddmig.index_handle = ddmid.index_handle
  INNER Join sys.tables AS t
    ON ddmid.OBJECT_ID = t.OBJECT_ID
WHERE 
  ddmid.database_id = DB_ID()
  AND CAST((ddmigs.user_seeks + ddmigs.user_scans) 
  * ddmigs.avg_user_impact AS INT) > 100
ORDER BY 
  CAST((ddmigs.user_seeks + ddmigs.user_scans) 
  * ddmigs.avg_user_impact AS INT) DESC;