public class Script
    {
      public static void Alert(string message)
        {
            ResponseScript("    alert('" + message + "');");
        }

        public static void ResponseScript(string script)
        {
            HttpContext.Current.Response.Write("<script type=/"text/javascript/">/n//<![CDATA[/n");
            HttpContext.Current.Response.Write( script );
            HttpContext.Current.Response.Write("/n//]]>/n</script>/n");
        }
    }

Script.AlertAndGoBack("溫馨提示：有重復的用戶！");

<script type="text/javascript">
//<![CDATA[
    alert('溫馨提示：有重復的用戶！');
    window.history.back();

//]]>
</script>

//將：
context.Response.ContentType = "text/plain";

//改為：
context.Response.ContentType = "text/html";