How to make a validation when user scan barcode twice

Question

Hi, currently i'm working on a lucky draw system. So for the registration part user need to scan their barcode into the system as an attendance for the event. How can i make a validation if user scan twice or more barcode at the same time. Thanks.

 

Code : 

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;



public partial class Attendance : System.Web.UI.Page
{
    string constr = ConfigurationManager.ConnectionStrings["lucky"].ConnectionString;


    protected void Page_Load(object sender, EventArgs e)
    {

        txtText.Focus();


    }


    protected void btnSave_Click(object sender, EventArgs e)
    {

        bool Idcheckbool = Idcheck();

        if (Idcheckbool)
        {
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("UPDATE Emp_Info SET Attendance = 'Present' WHERE  Emp_id= @id"))
                {
                    using (SqlDataAdapter sda = new SqlDataAdapter())
                    {
                        cmd.Connection = con;
                        sda.SelectCommand = cmd;
                        cmd.Parameters.AddWithValue("@id", txtText.Text);
                        using (DataTable dt = new DataTable())
                        {
                            sda.Fill(dt);
                            con.Open();
                            cmd.ExecuteNonQuery();
                            con.Close();
                            txtText.Text = string.Empty;

                        }
                    }

                }
            }
        }

    }

    public bool Idcheck()
    {

        string query = "select Emp_Name from Emp_Info where Emp_id='" + txtText.Text + "'";

        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand(query))
            {
                using (SqlDataAdapter sda = new SqlDataAdapter())
                {

                    cmd.Connection = con;
                    sda.SelectCommand = cmd;
                    using (DataTable dt = new DataTable())
                    {

                        sda.Fill(dt);
                        if (dt.Rows.Count > 0)
                        {
                            lblError.Text = "Welcome  "+ dt.Rows[0]["Emp_Name"].ToString();
                            Response.AddHeader("REFRESH", "4;URL=Attendance.aspx");
                            return true;

                        }

                        else
                        {
                            lblError.Text += "ID Doest Not Exist! ";
                            lblError.BackColor = System.Drawing.Color.Yellow;
                            Response.AddHeader("REFRESH", "3;URL=Attendance.aspx");
                            return false;

                        }

                    }
                }
            }
        }

    }




}