checkbox checked item add another list box c#

Question

i will getting the problem when add to checked item another list box 

 

this is add but it add repeat checked item

 

and this check box inside datalist

 

and the code write on check box OnCheckedChanged events.

 

 

i want checked item add into listbox my code is:

 

 

 

protected void OnCheckedChanged(object sender, EventArgs e)

 {

    foreach (DataListItem item in divProducts.Items)

{

     Label lbl_pro_nm = (Label)item.FindControl("lbl_pro_nm");

     CheckBox chkbox = (CheckBox)item.FindControl("chkbox");

  if (chkbox.Checked)

     {

         ListBox1.Items.Add(lbl_pro_nm.Text);

     }

else

    {

     ListBox1.Items.Remove(lbl_pro_nm.Text);

     }

     }

  }