how to delete the duplicate rows in ASP.Net GridView control

To what way student is useful Dropdownlist1(Good,Fair,Poor)   When i select the Dropdpownlist1 in that select the Poor POPUP Screen will open. the POPUP Screen as follows Negative_Feed_Back Textbox1(In that textbox type the reason for Poor) OK (Button)   When user click the OK (Button) the select dropdownlist and Remarks will be displayed in the gridview In gridview as follows Dropdown Remarks 0 Student performance is not good Suppose if user wrongly type the reason for poor want to change means in that case he can change reason for poor and click the ok button. In that case two Poor reasons are there in gridview as follows Dropdown Remarks 0 Student performance is not good 0 The Students not good at all in that case i want to delete the first First Row of the gridview.   Output as follows Dropdown Remarks 0 The Students not good at all   My code as follows

private void SetDropdowndetails(string Dropdownname, Int32 Dropdownid)
    {
         string SelectedDropdown = string.Empty;
        SelectedDropdown = Dropdownname.ToString();
        hfnegativefeedback.Value = Dropdownid.ToString();
        Negativepnlpopup();
    }
 
    private void Negativepnlpopup()
    {
       TxtNegativeFeedback.Text = "";
       DataTable dt = new DataTable();       
       dt = (DataTable)ViewState["Remarks"];
       if (ViewState["Remarks"] != null)
       {           
           for (int i = 0; i < dt.Rows.Count; i++)
           {
               if (hfnegativefeedback.Value == dt.Rows[i][0].ToString())
               {
                   TxtNegativeFeedback.Text = dt.Rows[i][1].ToString();
               }               
           }
       }
            RemoveDuplicaterows();
            ModalPopupExtender1.TargetControlID = SelectedDropdown.ToString();
            TxtNegativeFeedback.Focus(); 
            ModalPopupExtender1.Show();        
            Pnlnegativefeedback.Visible = true;
    }
 
    private void Insertdata()
    {
        DataTable dt = new DataTable();
        dt = (DataTable)ViewState["Remarks"];
        DataRow drow = dt.NewRow();
        drow[0] = hfnegativefeedback.Value;
        drow[1] = TxtNegativeFeedback.Text.ToString();
        dt.Rows.Add(drow);
        gvnegative.DataSource = dt; 
        gvnegative.DataBind();      
    }
 

    private void RemoveDuplicaterows()
    {
 
        DataTable dt = new DataTable();
        dt = (DataTable)ViewState["Remarks"];
        dt = dt.DefaultView.ToTable(true, "Dropdown", "Remarks");
        gvnegative.DataSource = dt;
        gvnegative.DataBind();
    }

  When i run my above code shows error as follows
  
 Column 'Remarks' does not belong to underlying table 'Table1'.
  
 
 The above error shows in the below function as follows
  

       DataTable dt = new DataTable();
        dt = (DataTable)ViewState["Remarks"];
        dt = dt.DefaultView.ToTable(true, "Dropdown", "Remarks");
        gvnegative.DataSource = dt;
        gvnegative.DataBind();

please help me.
what is the problem in my code

regards,
narasiman P.