arcobjects - Getting unique value records from a field

Building a desktop application using C#.net and ArcObjects. I'm trying to retrieve records from a shapefile using IQueryFilter. Having trouble writing the where clause in my scenario and the applicable logic.

Here is the scenario:

I have a 'Category' and "Completed" field, (both are of type string). What I need to do is, for a given category to see whether all the values in the completed field are unique. If values are not unique I need to display those records out.

Sample data

In the above example Category B and C does not have unique values. So all these records for B and C should be displayed.

I suppose a using an IQueryFilter interface should work. But puzzled on how I can achieve this by using a correct where clause. (I just need to get where the combination of Category and Completed fields are not unique)

Please help...