How to avoid the duplicate Entry ?

Question

HI To ALL,  

                I had a problem with duplication entry while trying to Place booking in Booking application.I have dealing with the Calltaxi Booking dispatch       application.Here the more than 200 endusers are working this application.when the people trying to press submit in the same time.That BookingNO Get
Duplicate. 

             Can anyone help me out if possible ...

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

                if (obj.BookingNo == "1")

                {

                    objA.BookingNo_Insert(obj.BookingNo, obj.BookingDate);

                }

                else

                {

                    BookingNo_Update(obj.BookingNo, obj.BookingDate);

                }

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

This Is The Function  which   I have used :

   public string BookingNo(DateTime rDate)

    {

        System.Globalization.DateTimeFormatInfo dateInfo = new System.Globalization.DateTimeFormatInfo();

        dateInfo.ShortDatePattern = "dd/MMM/yyyy";

       

        string dateformat = "dd/MMM/yyyy";

        string RequestDate = rDate.ToString(dateformat);

        //SqlConnection con = new SqlConnection(connString);

        //SqlCommand cmd = new SqlCommand();

        OleDbConnection con = new OleDbConnection(connGrid);

        OleDbCommand cmd = new OleDbCommand();

        con.Open();

        string Query = "select BOOKING_NO from TEST_bookingno where REQUEST_DT=to_date('" + RequestDate + "','dd/MON/yy')";

        //cmd = new SqlCommand(Query, con);

        //SqlDataReader Dr = cmd.ExecuteReader();

        cmd = new OleDbCommand(Query, con);

        OleDbDataReader Dr = cmd.ExecuteReader();

        int r;

        try

        {

            if (Dr.Read())

            {

                string d = Dr["BOOKING_NO"].ToString();

                r = Convert.ToInt32(d);

                r = r + 1;

                txtBookingNo.Text = r.ToString();

                //}

            }

            else

            {

                txtBookingNo.Text = Convert.ToString(1);//set the value in textbox which name is id

            }

        }

        catch (Exception ex)

        {

            BL_ErrorLog objLog = new BL_ErrorLog();

            objLog.Write(ex);

                

        }

        finally

        {

            con.Close();

        }

        return txtBookingNo.Text;

    }

 **********************************************************************************************

      Thanks in advance..

Regard ,

Ernesto Karthik