asp.net logic error-Multiple controls with the same ID 'cbk1' were found.

Please help me with the code.

error:

Multiple controls with the same ID 'cbk1' were found. FindControl requires that controls have unique IDs.

Code:

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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.Collections;




public partial class xDefault : System.Web.UI.Page
{
    public int tNUM;
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection cn = new SqlConnection("Data Source=ADMIN-B2C44FCB4\\SQLEXPRESS;Initial Catalog=examfeb8;Integrated Security=True");
        cn.Open();
        SqlCommand c = new SqlCommand("select * from stud where snum='" + Session["id"].ToString() + "'", cn);


        SqlDataReader r = c.ExecuteReader();


        r.Read();
        lblStuName.Text = r["sname"].ToString();
        Session["sname"] = lblStuName.Text;


        //string q = lblStuName.Text;


        cn.Close();
        DetailsView1.DataBind();
        getOption();




    }
    public void getOption()
    {
        int i = 1;
        SqlConnection con = new SqlConnection("Data Source=ADMIN-B2C44FCB4\\SQLEXPRESS;Initial Catalog=examfeb8;Integrated Security=True");
        con.Open();
        SqlCommand cmd1 = new SqlCommand("select top 1 * from Quest where Course='" + Session["Course"].ToString() + "'", con);
        SqlDataReader sdr = cmd1.ExecuteReader();
        while (sdr.Read())
        {
            RadioButtonList cbk = new RadioButtonList();
            cbk.ID = "cbk" + i.ToString();


            cbk.Items.Add("A. " + Server.HtmlEncode(sdr["opt1"].ToString()));
            cbk.Items.Add("B. " + Server.HtmlEncode(sdr["opt2"].ToString()));
            cbk.Items.Add("C. " + Server.HtmlEncode(sdr["opt3"].ToString()));
            cbk.Items.Add("D. " + Server.HtmlEncode(sdr["opt4"].ToString()));
            cbk.Font.Size = 11;
            for (int j = 1; j <= 4; j++)
            {
                cbk.Items[j - 1].Value = j.ToString();
            }
            Panel2.Controls.Add(cbk);
            i++;
            tNUM++;
        }
            sdr.Close();
            con.Close();



    }
    protected void btnNxt_Click(object sender, EventArgs e)
    {
        getOption();


        //---------
        string msc = "";
        for (int i = 1; i<= tNUM; i++)
        {
            RadioButtonList list = (RadioButtonList)Panel2.FindControl("cbk" + i.ToString());
            if (list != null)
            {
                if (list.SelectedValue.ToString() != "")
                {


                    msc += list.SelectedValue.ToString();
                }
                else
                {
                    msc += "0";
                }
            }
        }
        //--------




        //--------




        System.Data.DataRowView dr = (System.Data.DataRowView)DetailsView1.DataItem;


        // Create Answer object to save values
        Answer a = new Answer();
        a.QuestionID = dr["QuestionOrder"].ToString();
        a.CorrectAnswer = dr["CorrectAnswer"].ToString();
        //a.UserAns = answerDropDownList.SelectedValue.ToString();
        a.UserAns = msc;


        ArrayList al = (ArrayList)Session["AnswerList"];
        al.Add(a);


        Session.Add("AnswerList", al);


        if (DetailsView1.PageIndex == DetailsView1.PageCount - 1)
        {
            // Go to evaluate answers
            Response.Redirect("xresult.aspx");
        }
        else
        {
            DetailsView1.PageIndex++;
        }


        if (DetailsView1.PageIndex == DetailsView1.PageCount - 1)
        {
            btnNxt.Text = "Finished";
        }




    }
}

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please use CODE tags when posting code

By the way, I think your problem lies here :

Code:

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RadioButtonList list = (RadioButtonList)Panel2.FindControl("cbk" + i.ToString

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I'd have used JavaScript here to find a particular control. much easier and cleaner.