Copy data From one drop down to another

hi,
  I want to copy data from one drop down list to another when page loads. parent drop down list gets data from DB. When I run project no data is copied but when i change an item in parent drop down then data is copied. agian i change agian all data copied. I want only once data is copied when form loads. If any one knows the soluation please do reply. Code is below. Thanks in advance.
 
protected void Page_Load(object sender, EventArgs e)
    {
       
            for (int i = DropDownList1.Items.Count - 1; i >= 0; i--)
            {
                DropDownList2.Items.Add(DropDownList1.Items[i].Text);
            }
   ...