<script type="text/javascript" language="javascript">
function display()
{
debugger;
var list=window.document .getElementById ("ListBox1");
var text=list.options[list.options.selectedIndex].text;
var text1=list.options[list.options.selectedIndex].text;
if(text==text1)

window.open("addform.aspx","mypage","scrollbar=no,height=500,width=300,resize=0,menubar=0,location=0");
}
//page_load
ListBox1.Attributes.Add("ondblclick", "javascript:display();");