Column name appears more than once in the result column list

I am inserting checkbox (4-5 checkboxes) values into the database table which is inside Gridview.

So what happening here is,

There 4 columns and 20 rows. and I am checking each checkbox from each column and 19 rows are unchecked. So i am getting error message as

Column name 'ADD_FLAG' appears more than once in the result column list. Msg 264, Level 16, State 1, Line 1

Column name 'MODIFY_FLAG' appears more than once in the result column list. Msg 264, Level 16, State 1, Line 1

Column name 'VIEW_FLAG' appears more than once in the result column list. Msg 264, Level 16, State 1, Line 1

Column name 'DEL_FLAG' appears more than once in the result column list.

I dont know why this is happening, may be due to placing of code for that columns. Here is my code:-

public bool Save()
{
    try
    {
        for (int i = 0; i < GrdRights.RowsInViewState.Count; i++)
        {
            string strSQLMKEY = "SELECT WMS_User_Rights.MKEY  FROM WMS_User_Rights Inner JOIN WMS_Menu_Rights on " +
                                       "WMS_User_Rights.User_Id = WMS_Menu_Rights.Mkey " +
                                       "where WMS_User_Rights.User_Id='" + Hid_Selected_user.Value + "' " +
                                       "AND WMS_User_Rights.DELETE_FLAG = 'N'";
            if (GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("child_menu_mkey")].Text.Trim().ToString() == "0")
            {
                strSQLMKEY += " and MENU_MKEY='" + GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("MKEY")].Text.Trim() + "'";
            }
            else
            {
                strSQLMKEY += " and MENU_MKEY='" + GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("child_menu_mkey")].Text.Trim() + "'";
            }
            con.Open();
            SqlCommand cmdMKEY = new SqlCommand(strSQLMKEY, con);
            if (cmdMKEY.ExecuteScalar() != null)
            {
                strMode = "M";
                iMKey = Convert.ToInt32(cmdMKEY.ExecuteScalar());
            }
            else
            {
                strMode = "A";
                iMKey = 0;
            }
            con.Close();
            StringBuilder StrPubBldg = new StringBuilder();
            XmlWriter xw = XmlWriter.Create(StrPubBldg);
            xw.WriteStartElement("DocumentElement");
            {
                xw.WriteStartElement("WMS_Menu_Rights");
                if (GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("child_menu_mkey")].Text.Trim().ToString() == "0")
                {
                    xw.WriteElementString("MENU_MKEY", GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("MKEY")].Text.Trim());
                }
                else
                {
                    xw.WriteElementString("MENU_MKEY", GrdRights.Rows[i].Cells[GrdRights.Columns.GetColumnIndexByDataField("child_menu_mkey")].Text.Trim());
                }
                xw.WriteElementString("USER_ID", Hid_Selected_user.Value);
                xw.WriteElementString("DELETE_FLAG", "N");
                xw.WriteElementString("CREATION_DATE", System.DateTime.Now.ToString("dd/MM/yyyy hh:mm:ss"));
                for (int j = 0; j < GrdRights.RowsInViewState.Count; j++)
                {
                    bool str_checkadd = ((CheckBox)((GridDataControlFieldCell)GrdRights.RowsInViewState[j].Cells[4]).FindControl("ChkIDAdd")).Checked;
                    bool str_checkEdit = ((CheckBox)((GridDataControlFieldCell)GrdRights.RowsInViewState[j].Cells[5]).FindControl("ChkIDEdit")).Checked;
                    bool str_checkView = ((CheckBox)((GridDataControlFieldCell)GrdRights.RowsInViewState[j].Cells[6]).FindControl("ChkIDView")).Checked;
                    bool str_checkdel = ((CheckBox)((GridDataControlFieldCell)GrdRights.RowsInViewState[j].Cells[7]).FindControl("ChkIDDelete")).Checked;

                    xw.WriteElementString("ADD_FLAG", str_checkadd == true ? "Y" : "N"); // 
                    xw.WriteElementString("MODIFY_FLAG", str_checkEdit == true ? "Y" : "N");
                    xw.WriteElementString("VIEW_FLAG", str_checkView == true ? "Y" : "N");
                    xw.WriteElementString("DEL_FLAG", str_checkdel == true ? "Y" : "N");                        
                }
            }
            xw.WriteEndElement();
            xw.Close();

            MainEnqMkey = InsertUpdateDelete.InsertUpdateDeleteCls.InsertUpdateDelete_sql(strMode, Convert.ToInt16(iMKey), "WMS_Menu_Rights", "MKEY", "MUR", StrPubBldg.ToString());

        }

        if (MainEnqMkey.Equals(0))
        {
            ClientScript.RegisterStartupScript(this.GetType(), "SuccessScript", "alert('Some Error Occured While Saving Data !!')", true);
        }
        else
        {
            con.Open();
            SqlCommand ObjPriCmd = new SqlCommand("delete from WMS_User_rights where MKEY=" + HidTempMkey.Value, con);
            ObjPriCmd.ExecuteNonQuery();
            con.Close();

            if (!Directory.Exists(Server.MapPath(StrFolder)))
            {
                Directory.CreateDirectory(Server.MapPath(StrFolder));
            }
            if (File.Exists(Server.MapPath(StrFolder + StrFileName)) == false)
            {
                using (System.IO.StreamWriter sw = File.CreateText(Server.MapPath(StrFolder + StrFileName)))
                {
                    sw.WriteLine("\n");
                    //sw.Write("CEF No. : " + TxtCefNo.Value.ToString().Trim() + " Followup Sr No :" + TxtSrNo.Value.ToString().Trim());
                    //sw.WriteLine("\n"); sw.WriteLine("\n");
                    //sw.Write("Mode : " + strMode + " & Xml : " + StrPubBldg.ToString());
                    sw.WriteLine("\n"); sw.NewLine = "\n------------------------------------------------------------------";
                    sw.WriteLine("\n");
                    sw.Close();
                    sw.Dispose();
                }
            }
            else
            {
                using (System.IO.StreamWriter sw = File.AppendText(Server.MapPath(StrFolder + StrFileName)))
                {

                    sw.WriteLine("\n");
                    //sw.Write("CEF No. : " + TxtCefNo.Value.ToString().Trim() + " Followup Sr No :" + TxtSrNo.Value.ToString().Trim());
                    //sw.WriteLine("\n");
                    //sw.Write("Mode : " + strMode + " & Xml : " + StrPubBldg.ToString());
                    sw.WriteLine("\n"); sw.NewLine = "------------------------------------------------------------------";
                    sw.WriteLine("\n");
                    sw.Close();
                    sw.Dispose();
                }
            }
        }
        return true;
    }
    catch (Exception ex)
    {
        if (!Directory.Exists(Server.MapPath(StrFolder)))
        {
            Directory.CreateDirectory(Server.MapPath(StrFolder));
        }
        if (File.Exists(Server.MapPath(StrFolder + StrFileName)) == false)
        {
            using (System.IO.StreamWriter sw = File.CreateText(Server.MapPath(StrFolder + StrFileName)))
            {
                sw.WriteLine("\n");
                //sw.Write("CEF No. : " + TxtCefNo.Value.ToString().Trim() + " Followup Sr No :" + TxtSrNo.Value.ToString().Trim());
                //sw.WriteLine("\n"); sw.WriteLine("\n");
                //sw.Write("Mode : " + strMode + " & Xml : " + StrPubBldg.ToString());
                sw.WriteLine("\n"); sw.NewLine = "\n------------------------------------------------------------------";
                sw.WriteLine("\n");
                sw.Close();
                sw.Dispose();
            }
        }
        else
        {
            using (System.IO.StreamWriter sw = File.AppendText(Server.MapPath(StrFolder + StrFileName)))
            {

                sw.WriteLine("\n");
                //sw.Write("CEF No. : " + TxtCefNo.Value.ToString().Trim() + " Followup Sr No :" + TxtSrNo.Value.ToString().Trim());
                //sw.WriteLine("\n");
                //sw.Write("Mode : " + strMode + " & Xml : " + StrPubBldg.ToString());
                sw.WriteLine("\n"); sw.NewLine = "------------------------------------------------------------------";
                sw.WriteLine("\n");
                sw.Close();
                sw.Dispose();
            }
        }
        return false;
    }

    finally
    {

    }

}

protected void CmdSave_Click(object sender, EventArgs e)
{
    if (Save() == true)
    {
        fillGrid();
        if (strMode == "M")
        {
            ClientScript.RegisterStartupScript(this.GetType(), "CloseScript", "alert('Record Modified Successfully');window.location.href='Frm_User_Rights.aspx?TranType=MUR&Mode=A&Key=0&PView=N&userid=" + Request.QueryString["userid"].ToString() + "';", true);
        }
        else
        {
            ClientScript.RegisterStartupScript(this.GetType(), "CloseScript", "alert('Record Saved Successfully');window.location.href='Frm_User_Rights.aspx?TranType=MUR&Mode=A&Key=0&PView=N&userid=" + Request.QueryString["userid"].ToString() + "';", true);
        }
    }
}

Also for database i am using SQL -server- 2005