To check if values are duplicate or not

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

public partial class _Default : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(@"Data Source=NEENA-PC\SQLEXPRESS;Initial Catalog=practice;Integrated Security=True");
    
    public void custom()
    {
        string select = "Select name from [insert]";
        SqlCommand cmd=new SqlCommand(select,con);
        SqlDataReader sdr;
        con.Open();
        sdr = cmd.ExecuteReader();
        //if (!IsPostBack)
        //{
            if (sdr.Read())
            {
                if (TextBox1.Text == sdr[0].ToString())
                {
                    Label1.Text = "error";
                    TextBox1.Text = "";
                    TextBox1.Focus();
                    //CustomValidator1.ErrorMessage = "erroe";
                }
                else
                {
                    Label1.Text = "ok";
                    //CustomValidator1.ErrorMessage = "scd";
                }
            }
       // }
        con.Close();
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        custom();
        Response.Write("Hiii");
    }
}