System.out.println("已经插入");
request.getRequestDispatcher("/login_success.jsp").forward(request, response);

<script type="text/javascript">
 window.onload = function(){ 
  //获取按钮的对象
  var btn = document.getElementById("btn");
  //为按钮绑定单击响应函数
  btn.onclick = function(){  
   //点击以后使按钮不可用
   this.disabled=true; 
   //当将提交按钮设置为不可用时，会自动取消它的默认行为
   //手动提交表单
   this.parentNode.submit(); 
  };
 };
</script><br><br><form action="${pageContext.request.contextPath }/SessionServlet" ><br>user:<input type="text" name="username"><br>password<input type="password" name="pwd"><br><input type="submit" value="提交" id="btn"> <br></form>

<%
 String uuid = UUID.randomUUID().toString();
 session.setAttribute("uuid", uuid);
%><br>${errormsg }<br><form action="${pageContext.request.contextPath }/SessionServlet" ><br>　　<input type="text" name="uuid" value="<%=uuid %>"/><br>　　user:<input type="text" name="username"><br>　　password<input type="password" name="pwd"><br>　　<input type="submit" value="提交" "><br></form>

String reqUUID = request.getParameter("uuid");
HttpSession session = request.getSession();
String sessUUID = (String) session.getAttribute("uuid");
session.removeAttribute("uuid");
  
if(reqUUID.equals(sessUUID)){
 response.sendRedirect(request.getContextPath()+"/login_success.jsp");
 System.out.println("已经插入");
}else{
 request.setAttribute("errormsg", "重复登陆");
 request.getRequestDispatcher("/3.jsp").forward(request, response);
}