[RESOLVED] A column named 'Request_ID' already belongs to this DataTable.

Getting error in code . 

  protected void search_Click(object sender, EventArgs e)
    {
        con.Open();
        SqlCommand q = new SqlCommand("select Request_ID, Tbl_Request.Request_Number,Tbl_Request.Request_Qty, Tbl_Device.Device_Name,Dept_Name.Dept_name,Tbl_Request.Request_user, Tbl_Request.Request_DeviceID,Tbl_Request.Request_Dept FROM Request where Request_Number=@Request_Number");
        q.Connection = con;
        q.Parameters.Add("@Request_Number", SqlDbType.BigInt).Value = Req.Text;
        SqlDataReader reader = q.ExecuteReader();
        if (reader.HasRows)
        {
            int count = 0;
             dt.Columns.Add("Request_ID");   <<<<<------ ERROR
            dt.Columns.Add("Request_Number");
            dt.Columns.Add("Request_Qty");
            dt.Columns.Add("Device_Name");
            dt.Columns.Add("Dept_name");
            dt.Columns.Add("Request_user");
            dt.Columns.Add("Request_DeviceID");
            dt.Columns.Add("Request_Dept");
            while (reader.Read())
            {
                    count++;
                    DataRow row = dt.NewRow();
                    row["Request_ID"] = reader.GetValue(0).ToString();
                    row["Request_Number"] = reader.GetValue(1).ToString();
                    row["Request_Qty"] = reader.GetValue(2).ToString();
                    row["Device_Name"] = reader.GetValue(3).ToString();
                    row["Dept_name"] = reader.GetValue(4).ToString();
                    row["Request_user"] = reader.GetValue(5).ToString();
                    row["Request_DeviceID"] = reader.GetValue(6).ToString();
                    row["Request_Dept"] = reader.GetValue(7).ToString();
                    dt.Rows.Add(row);  

            }
        }

Do you have global dt?

You can not add column everytime. Try like this

DataTable dt = New DataTable();
dt.Columns.Add("Request_ID");
dt.Columns.Add("Request_Number");
dt.Columns.Add("Request_Qty");
dt.Columns.Add("Device_Name");
dt.Columns.Add("Dept_name");
dt.Columns.Add("Request_user");
dt.Columns.Add("Request_DeviceID");
dt.Columns.Add("Request_Dept");

use this:

int count = 0;

dt.Columns.Clear();

dt.Columns.Add("Request_ID"); <<<<<------ ERROR

dt.Columns.Add("Request_Number");

dt.Columns.Add("Request_Qty");

dt.Columns.Add("Device_Name");

dt.Columns.Add("Dept_name");

dt.Columns.Add("Request_user");

dt.Columns.Add("Request_DeviceID");

dt.Columns.Add("Request_Dept");