How to test if a session already exists?
In Symfony, session service is an instance of Symfony\Component\HttpFoundation\Session\Session. That said, session does provide several method to either check or manipulate session.
In your particular case, you would do something like this:
$session = $request->getSession();
if ( $session->has('login')){
$message = 'Bonjour '.$session->get('login').' vous etes connecté.<br>';
}
But, noting the comment from @Gerry, his question is a very valid one.
Hope this helps...