session()->get() only understands keys. You can't pass an array to it to check if the user exists. How did you create the session in the first place? You should have given that session a key. You can then retrieve the session using that key again.
Also you can use the has function here ;)
if (session()->has($key)) {
$this->auth->logout(); //this to logout specific user
return redirect()->to(route('login'));
}