Prevent duplicate item from being added to a GridView using ASP.NET C#

Question

I have this code for adding items into gridview. But in my below code duplicate items are not added. I need distinct items with no duplicate entries.

Below is my .cs code.

   private DataTable list(String dbObject, String filterName, String filterValue,string PositonId,string Status)
{
    NameValuePairList objNameValuePairList = new NameValuePairList();
    objNameValuePairList.Add(new NameValuePair("@FilterValue", filterValue, PositonId, Status));
    objNameValuePairList.Add(new NameValuePair("@Action", "FilterBy" + filterName, PositonId, Status));
    DataTable dt = dl.Search_RegisterationInfo(dbObject, objNameValuePairList, PositonId, Status);
    return dt;
}

public DataTable list(String dbOject, FilterList myFilterList,string PositonId,string Status)
{
    // gets a collection(dataset) of all unique filters(datatables) and also group all subfilters(rows) under each filter
    DataTable dt;
    DataSet ds = new DataSet();
    // a filter may be a Nationality or a Qualification 
    foreach (Filter item in myFilterList)
    // a subfilter may be Indian or Expatriate under the filter Nationality 
    {
        // another subfilter may be Bachelor degree or Master Degree under the filter Qualification
        dt = list(dbOject, item.getFilterName, item.getFilterValue, PositonId,Status);
        dt.TableName = item.getFilterName;
        // datatables are named based on the filters
        if (ds.Tables.Count == 0)
            // so we get a collection of unique filters (datatables) in the dataset
            ds.Tables.Add(dt);
        // add new filter without checking, since for the first time, no conflicts are possible
        else
        {
            bool tableMatchFound = false;
            foreach (DataTable newdt in ds.Tables)
                if (newdt.TableName == dt.TableName)
                {
                    // see if filter is already present in the dataset
                    tableMatchFound = true;
                    // when the current filter is already present in the dataset
                    foreach (DataRow dr in dt.Rows)
                        ds.Tables[newdt.TableName].ImportRow(dr);

                }
            // importrow() adds distinct new subfilters to the existing filter, duplicate items are not added
            if (!tableMatchFound)
                ds.Tables.Add(dt);
        }
        // if the filter does not exist, add the new filter to the collection
    }
    // the entire collection of filters will contain duplicate items
    // distinct items from the entire collection is filtered out in the next section
    dt = ds.Tables[0].Clone();
    // get the structure of the first filter as they all apply to the same table object  
    if (ds.Tables.Count == 1)
        dt = ds.Tables[0];
    // if there is only one filter, no filtering is required
    else
        // if there are more than one, compare each subfilter of every other filter with the subfilters of the first filter
        foreach (DataRow dr in ds.Tables[0].Rows)
        {
            // each subfilter from the first filter is used as a pivot
            int rowMatchFound = 1;
            for (int i = 1; i < ds.Tables.Count; i++)
                // search all filters except the first one
                foreach (DataRow newdr in ds.Tables[i].Rows)
                    // select each subfilter from all the filter
                    if ((int)dr["RegistrationId"] == (int)newdr["RegistrationId"])
                        rowMatchFound++;
            if (rowMatchFound == ds.Tables.Count)
                // a match is found exactly once in all the filters
                dt.ImportRow(dr);
            // the final item is selected so that is is present in all the filters   
        }
    return dt;
}

For more details, I am using below example code:

http://www.codeproject.com/Tips/773362/Filtering-search-results-with-multiple-CheckBox-Li

There is an above comment line saying clearly in my question like:

// importrow() adds distinct new subfilters to the existing filter, duplicate items are not added

Thank you in advance