Graph returned an error: This authorization code has been used error thrown when refreshing the page after login using facebook Id

This authorization code has been used error retued when refreshing the page after login using facebook id. I have read many blogs and I did not get any idea about this issue. Here is my code.

fblogin.php

<?php

session_start();
// added in v4.0.0
require_once __DIR__.'vendorautoload.php';

//require 'functions.php';  

use FacebookFacebookSession;
use FacebookHelpersFacebookRedirectLoginHelper;
use FacebookFacebookRequest;
use FacebookFacebookResponse;
use FacebookExceptionsFacebookSDKException;
use FacebookFacebookRequestException;
use FacebookFacebookAuthorizationException;
use FacebookGraphNodesGraphObject;
use FacebookAuthenticationAccessToken;
use FacebookHttpClientsFacebookCurlHttpClient;
use FacebookHttpClientsFacebookHttpable;

$appId='myappid';
$secretkey='mysecretkey';
// init app with app id and secret
FacebookSession::setDefaultApplication($appId,$secretkey);


$fb=new FacebookFacebook(['app_id'=>'myappid','app_secret'=>'mysecretkey','default_graph_version' => 'v2.4']);

// login helper with redirect_uri
//$helper = new FacebookRedirectLoginHelper('http://localhost:81/fblogin/fblogin.php',NULL,NULL);
$helper=$fb->getRedirectLoginHelper();
$permissions = ['email', 'user_likes'];    

if(isset($_REQUEST['logout'])){

    //$accessToken=null;
    unset($_SESSION['facebook_access_token']);
    unset($_SESSION['facebook_user']);
    session_unset();
}

try {

  // Logged in!
  $accessToken = $helper->getAccessToken();
      } catch(FacebookExceptionsFacebookResponseException $e) {
  // When Graph retus an error
  echo 'Graph retued an error: ' . $e->getMessage();
  echo 'Facebook SDK retued an error: ' . $e->getTraceAsString();
} catch(FacebookExceptionsFacebookSDKException $e) {
  // When validation fails or other local issues
  echo 'Facebook SDK retued an error: ' . $e->getMessage();
  echo 'Facebook SDK retued an error: ' . $e->getTraceAsString();

}

 if (isset( $accessToken)) {
              $oAuth2Client = $fb->getOAuth2Client();
              $longLivedAccessToken = $oAuth2Client->getLongLivedAccessToken($accessToken);
              $_SESSION['facebook_access_token'] = (string) $accessToken;

                    if(!isset($_SESSION['facebook_user'])) {
                    $response=$fb->get('/me?fields=id,name',$_SESSION['facebook_access_token']);

                    $user=$response->getGraphUser();

                      $_SESSION['facebook_user']=$user;

              }
        } elseif ($helper->getError()) {
          // The user denied the request
          exit;
        }
        else{
            $loginUrl=$helper->getLoginUrl('http://localhost:81/fblogin/fblogin.php',$permissions,'&');
        }
include 'index.php';
?>

index.php

<?php //
//session_start(); 

if(isset($_SESSION['facebook_user']))
{
    if($_SESSION['facebook_user'])!
    print_r($_SESSION['facebook_user']);
    echo '<a href=?logout>Logout</a>';
}
else
{
    echo '<a href="'.$loginUrl.'">Login with FB</a>';
}
?>

Steps:
-Run fblogin.php and click "Login with FB" link
-Enter user name and password of FB
-Check User id and name is displayed with Logout link in Index.php
-Now refresh the browser.
-Verify "Graph retued an error: This authorization code has been used" error thrown.

My question is, to resolve this problem, what i will do with the above code. If I have missed anything in my code, suggest me where i want to add or remove the line of code.

I am using Wamp server and Facebook SDK 5. Thanks in advance.