Api Auth just one Login each User

Hello,
i got your plugin running with a c# app.
Till now i can login over http://localhost/api/user/generate_auth_cookie/?username=john&password=PASSWORD-HERE
.

How i can use it so that just one user get just one time logged and has no chance to loggin with api again, when he is logged?

Someting like that add to?
Have you an example for me?

function is_user_logged_in() {
    $user = wp_get_current_user();
 
    return $user->exists();
}

Thx for help