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("温馨提示：有重复的用户！");

script type="text/javascript">
//![CDATA[
    alert('温馨提示：有重复的用户！');
    window.history.back();

//]]>
/script>

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

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