ListBox depulicates on page fire?

[nebrom]

i am using C#,asp.net and sql server 2005.
now i want to show the value of one column in listbox during page load.
i did as follow:

 string sqlvisa = "select visano from saudivisatbl";
        con.Open();
        SqlCommand sqlquery = new SqlCommand();
        DataTable data = null;
        sqlquery.Connection = null;
        SqlDataAdapter ddapter = null;
        sqlquery.CommandText = sqlvisa;
        sqlquery.Connection = con;
        data = new DataTable();
        ddapter = new SqlDataAdapter(sqlquery);
        ddapter.Fill(data);
        for (int i = 0; i < data.Rows.Count; i++)
        {
            string data1 = data.Rows[i]["visano"].ToString();
            listboxvisa.Items.Add(data1);

        

        }
        con.Close();

this code displays me correctly the value on list box.
now i want to assign the value of the selected value in the listbox into textbox1 on button click

protected void btnselectvalue_Click(object sender, EventArgs e)
    {

  textbox1.Text = listboxvisa.SelectedItem.ToString();
    }

again the the value of the selected list box item is dispayed in the textbox1 on btn click, but the value of the listboxvisa becomes duplicated, when i click the button then again it will duplicate,
how do i prevent this duplication of on button click.
thanks