Hi Steve, this is a known issue; the EventTarget and EventArgument fields aren't cleared when using the Back Button of the browser. This means that they are going to be posted back to the server again. The work-around for this issue is to clear those fields before the form is submitted; something like this:
<head runat="server">
<script>
function clearValues()
{
theForm.__EVENTTARGET.value ='';
return true;
}
</script>
</head>
<body>
<form id="form1" runat="server" onsubmit="return clearValues();">
Hope this helps,
Joe