Per session per user!

add_filter( 'authenticate','one_session_per_user', 30, 3 );

function one_session_per_user( $user, $username, $password ) {
$sessions = WP_Session_Tokens::get_instance( $user->ID );
$all_sessions = $sessions->get_all();
if ( count($all_sessions) ) {
$user = new WP_Error(‘already_signed_in’, __(‘<strong>ERROR</strong>: User already logged in.’));
}
return $user;
}