public void addUser(User u){
        boolean exists = false;
        User existingUser = null;
        LinkedList<User> userCopy = new LinkedList<>();
        for(User usr : users){
            userCopy.add(usr);
        }
        for(User user : userCopy){
            if(user.getIP().getHostName().equals(u.getIP().getHostName())){
                exists = true;
                existingUser = user;
            }
        }
        if(exists){
            this.users.remove(existingUser);
            existingUser.setLastSeen(Timestamp.getCurrentTimeAsLong());
            this.users.add(existingUser);
        }else{
            this.users.add(u);
        }
        updateGUI();
    }