public function add($tracking)
    {
        $sessionTracks = [];
        if (Session::has($this->sessionIdentifier)) {
            $sessionTracks = Session::get($this->sessionIdentifier);
        }

        //	prevent duplicates in session
        $trackingKey = md5($tracking);
        $sessionTracks[$trackingKey] = $tracking;

        Session::flash($this->sessionIdentifier, $sessionTracks);
    }