back button goes to confirm form resubmission

Question

I made a logout servlet for logout button, when clicking on logout servlet it successfully going to login page as coded. But when clicking on back button, it asks for form resubmission and after confirming it again going to previous user session.

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    /* PrintWriter out = response.getWriter(); */
    
    HttpSession session = request.getSession();
    response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate, max-age=0, post-check=0, pre-check=0");
    response.setHeader("Cache-control", "no-cache");
    response.setHeader("Pragma", "no-cache");
    response.setHeader("Expire","0");
    response.setDateHeader("Expires",-1);
    session.invalidate(); 

    String userr = (String)request.getAttribute("k");
    if (userr == null)
        response.sendRedirect("Login.html");
}