[Wiki] How to prevent Login from two places?

/**
     * session_validate()
     * Will check if a user has a encrypted key stored in the session array.
     * If it returns true, user is the same as before
     * If the method returns false, the session_id is regenerated
     *
     * @param {String} $email   The users email adress
     * @return {boolean} True if valid session, else false
     */
 
    public function session_validate(  )
    {
 
        // Encrypt information about this session
        $user_agent = $this->session_hash_string($_SERVER['HTTP_USER_AGENT'], $this->user_email);
 
        // Check for instance of session
        if ( session_exists() == false )
        {
            // The session does not exist, create it
            $this->session_reset($user_agent);
        }
 
        // Match the hashed key in session against the new hashed string
        if ( $this->session_match($user_agent) )
        {
            return true;
        }
 
        // The hashed string is different, reset session
        $this->session_reset($user_agent);
        return false;
    }
 
    /**
     * session_exists()
     * Will check if the needed session keys exists.
     *
     * @return {boolean} True if keys exists, else false
     */
 
    private function session_exists()
    {
        return isset($_SESSION['USER_AGENT_KEY']) && isset($_SESSION['INIT']);
    }
 
    /**
     * session_match()
     * Compares the session secret with the current generated secret.
     *
     * @param {String} $user_agent The encrypted key
     */
 
    private function session_match( $user_agent )
    {
        // Validate the agent and initiated
        return $_SESSION['USER_AGENT_KEY'] == $user_agent && $_SESSION['INIT'] == true;
    }
 
    /**
     * session_encrypt()
     * Generates a unique encrypted string
     *
     * @param {String} $user_agent      The http_user_agent constant
     * @param {String} $unique_string    Something unique for the user (email, etc)
     */
 
    private function session_hash_string( $user_agent, $unique_string )
    {
        return md5($user_agent.$unique_string);
    }
 
    /**
     * session_reset()
     * Will regenerate the session_id (the local file) and build a new
     * secret for the user.
     *
     * @param {String} $user_agent
     */
 
    private function session_reset( $user_agent )
    {
        // Create new id
        session_regenerate_id(TRUE);
        $_SESSION = array();
        $_SESSION['INIT'] = true;
 
        // Set hashed http user agent
        $_SESSION['USER_AGENT_KEY'] = $user_agent;
    }
 
    /**
     * Destroys the session
     */
 
    private function session_destroy()
    {
        // Destroy session
        session_destroy();
    }