The way you would do this in the GridView is to look at each row as it is inserted, and check to see if the row is a duplicate of any other row (in a column by column comparison). It isn't pretty, but it is the best there is once you get to this point. Here is a couple of links that will help you with the code:
http://social.msdn.microsoft.com/Forums/en-AU/csharpgeneral/thread/d4ef2cfd-99e0-43e8-a274-d7bf2568d494[
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http://stackoverflow.com/questions/8717898/datagridview-find-duplicate-rows-and-update-existing-data[
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The better way of doing this is to remove the duplicates before you get the data to the GridView. Use a better SQL query (such as SELECT DISTINCT) to get your data. This way you can load your data directly without the expensive overhead of looping through each record. Also, since you did your duplicate removal at the database server, you will have transmitted less data to the client, thus saving yourself even more overhead.
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