When I Search The Gridview Record It Repeated Infinity Times

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;
using System.Configuration;
using System.IO;
using System.Data;
using AjaxControlToolkit;

namespace Virtual_Classroom.Student
{
    public partial class Inbox : System.Web.UI.Page
    {

       
        protected void Page_Load(object sender, EventArgs e)
        {
           if (!this.IsPostBack)
    {
        this.BindGrid();
    }
        }
private void BindGrid()
{
    string constr = ConfigurationManager.ConnectionStrings["VCRConnectionString"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
             cmd.CommandText="SELECT ReplyDoubtID,Name,Subject,ReplyDoubt,Date,Files,FileName,Topic, ISNULL((SELECT AVG(Rating) FROM Rates WHERE ReplyDoubtID = ReplyDoubt.ReplyDoubtID), 0) Rating FROM ReplyDoubt";
            
             
            cmd.Connection = con;
            
            DataTable dt = new DataTable();
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                sda.Fill(dt);
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
        }
    }
}
 

        protected void LinkButton1_Click(object sender, EventArgs e)
        {

        }

        protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
        {
            if (e.CommandName == "Download")
            {
                Response.Clear();
                Response.ContentType = "application/octet-stream";
                Response.AppendHeader("Content-Disposition", "filename=" + e.CommandArgument);
                Response.TransmitFile(Server.MapPath("~/Files/") + e.CommandArgument);
                Response.End();
            }
        }

        protected void OnRatingChanged(object sender, RatingEventArgs e)
        {
            int rowIndex = ((sender as Rating).NamingContainer as GridViewRow).RowIndex;
            int ReplyDoubtID = Convert.ToInt32(GridView1.DataKeys[rowIndex].Value);
            string constr = ConfigurationManager.ConnectionStrings["VCRConnectionString"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("INSERT INTO Rates VALUES(@ReplyDoubtID, @Rating)"))
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Parameters.AddWithValue("@ReplyDoubtID", ReplyDoubtID);
                    cmd.Parameters.AddWithValue("@Rating", e.Value);
                    cmd.Connection = con;
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
            }
            
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
           string constr = ConfigurationManager.ConnectionStrings["VCRConnectionString"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = " SELECT * FROM ReplyDoubt,Rates WHERE Topic LIKE '%' + @Topic + '%'"; ;
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@Topic", TextBox3.Text.Trim());
            DataTable dt = new DataTable();
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                sda.Fill(dt);
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
        }
    }
        }

        protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            BindGrid();
            GridView1.PageIndex = e.NewPageIndex;
            GridView1.DataBind();
        }

       
    }
}