javascript call

Hi,

For some reason, the code is only "accurate" for the first time you run it. After the second time, it is showing the value that submitted from the previous time, kind of confused.  thanks

    <script type = "text/javascript">
        function Confirm() {
            var confirm_value = document.createElement("INPUT");
            confirm_value.type = "hidden";
            confirm_value.name = "confirm_value";
            if (confirm("Report name exists already. Do you want to replace this report?")) {
                confirm_value.value = "Yes";
            } else {
                confirm_value.value = "No";
            }
            document.forms[0].appendChild(confirm_value);
        }
    </script>

 <asp:Button id="SaveReportbtn" onclick="SaveReportbtn_Click"  CustomParameter="listBox1" runat="server" text="Save Report" />Report" />

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                    ScriptManager.RegisterStartupScript(this, GetType(), "myFunction", "Confirm();", true);     

                    string confirmValue = Request.Form["confirm_value"];
                    if (confirmValue == "Yes")
                    {
                        this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked YES!')", true);
                     
                    }
                    else
                    {
                        this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked NO!')", true);
                     


                    }

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