Attendance Project : When i pressed mark attendance saved many times i want only everyday attendance

Attendance Project : When i pressed mark attendance saved many times i want only everyday attendance please solve this send code

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

public partial class Attendance : System.Web.UI.Page
{
    string strcon = ConfigurationManager.ConnectionStrings["rkcomputers"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {
        DateTime utcNow = DateTime.UtcNow;
        TimeZoneInfo indiaTimeZone = TimeZoneInfo.FindSystemTimeZoneById("India Standard Time");
        DateTime indiaTime = TimeZoneInfo.ConvertTimeFromUtc(utcNow, indiaTimeZone);
        Label1.Text = indiaTime.ToString();
    }


    protected void Button1_Click(object sender, EventArgs e)
    {
        if (checkstudentexist())
        {
            Response.Write("<script>alert('Already Student Attendance SAVED for TODAY...');</script>");
        }

        //else if (checkstudentexist() == (checkstudentexist11to12()))
        // {

        // Response.Write("<script>alert('Attendance Already Completed');</script>");
        // }
        else
        {
            studentattendance();
            Response.Write("<script>alert('Student Attendance SAVED for TODAY...');</script>");
        }

        //ScriptManager.RegisterStartupScript(this, this.GetType(), "Redit", "window.location='" + Request.ApplicationPath + "regform.aspx';", true);
    }

    private void saveattendance(int admissionno, string studentname, string dateofclass1, string status, string sclass)
    {
        string query = " insert into StudentAttendance(admissionno,studentname,dateofclass,attendancestatus,class) values(" + admissionno + ",'" + studentname + "','" + dateofclass1 + "', '" + status + "','" + sclass + "')";

        SqlConnection con = new SqlConnection(strcon);
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = query;
        cmd.Connection = con;
        cmd.ExecuteNonQuery();
    }

    bool checkstudentexist()
    {
        try
        {
            SqlConnection con = new SqlConnection(strcon);
            if (con.State == ConnectionState.Closed)
            {
                con.Open();
            }

            SqlCommand cmd = new SqlCommand("SELECT* from StudentAttendance where dateofclass = '" + Label1.Text + "'", con);

            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);

            if (dt.Rows.Count >= 1)
            {
                return true;
            }
            else
            {
                return false;
            }
        }
        catch (Exception ex)
        {
            Response.Write("<script>alert('" + ex.Message + "');</script>");
            return false;
        }
    }


    void studentattendance()
    {
        try
        {
            foreach (GridViewRow row in GridView1.Rows)
            {
                int admissionno1 = Convert.ToInt32(row.Cells[0].Text);
                string studentname1 = row.Cells[1].Text;
                RadioButton rbtn1 = (row.Cells[2].FindControl("RadioButton1") as RadioButton);
                RadioButton rbtn2 = (row.Cells[2].FindControl("RadioButton2") as RadioButton);
                string status1;
                if (rbtn1.Checked)
                {
                    status1 = "Present";
                }
                else
                {
                    status1 = "Absent";
                }
                DateTime utcNow = DateTime.UtcNow;
                TimeZoneInfo indiaTimeZone = TimeZoneInfo.FindSystemTimeZoneById("India Standard Time");
                DateTime indiaTime = TimeZoneInfo.ConvertTimeFromUtc(utcNow, indiaTimeZone);

                string dateofclass1 = indiaTime.ToString();
                string sclass1 = DropDownList1.SelectedItem.Text;
                saveattendance(admissionno1, studentname1, dateofclass1, status1, sclass1);
                //Label2.Text = "Attendance HAVE Been Completed...";
            }
        }
        catch (Exception Ex)
        {
            // Response.Write("<script>alert('" + ex.Message + "');</script>");

        }
    }
}