title: Asp .net dropdownlist data keep load
body: <p>After I get the value show in GridView the DropDownList will contain duplicate items when I click again.</p>
  
  <pre><code>public void Page_Load(object sender, EventArgs e)
  {
      string sql = "select distinct cproject from I.dd.project";
      con.Open();
      SqlCommand cmd = new SqlCommand(sql, con);
      SqlDataReader dr = cmd.ExecuteReader();
      while (dr.Read())
      {
         DropDownList1.Items.Add(dr[0].ToString());
      }
      con.Close();
  }
  
  public void button_click(object sender, EventArgs e)
  {
      sqldataadapter da = new sqldataadapter(Select * from lalala where id =    '"+dropdownlist.item.selectedvalue.tostring()+"')
     +"where A.cproject ='"+DropDownList1.SelectedValue.ToString()+"', con);
      DataSet ds = new DataSet();
      sda.Fill(ds);
      GridView1.DataSource = ds;
      GridView1.DataBind();
  }
  </code></pre>