SQL Server Index recomended but already there (DMVs)
SQL Server 2008 and 2008 R2 have known issues in the index-recommendation code that were fixed in SQL Server 2012. 

Having said that, index recommendations should always be looked at carefully to see if there is (1) an existing index that might be slightly modified to handle it's current load along with the new load, and (2) if any changes can be made to the query producing the recommendation so it uses an existing index.

Paul Randal wrote about the bug in SQL Server 2008 here.