Session variables missing after fail logging in.

So, I have this middleware called CheckOneUser it has this code.

public function handle($request, Closure $next)
    {
        $response = $next($request);

        return $response;
    }

    public function terminate($request, $response)
    {
        if (! \Auth::check())
            return;

        $user = \Auth::user();

        if (!$user->session_id) {
            $user->session_id = \Session::getId();
            $user->last_activity = Carbon::now();
            $user->save();
        }
        else {
            $last_session = \Session::getHandler()->read($user->session_id);

            if ($last_session) {
                \Auth::logout();
                \Session::getHandler()->destroy(\Session::getId());
                \Session::flash('session', 'Someone already logged in.');
                return redirect()->route('login', ['session' => 'Someone already logged in.']);
            }
        }
    }

I want to happen is that when the user happens to login then someone is already logged in. I will return a message to my view. My view has this line where I can check if it has session.

                        {{session()->has('session') ? session()->get('session') : 'sadsa'}}

And my web.php looks like this.

Route::group(['middleware' => ['checkoneuser']], function () {
    Route::post('login', 'Auth\LoginController@login');

    Route::get('login', 'Auth\LoginController@showLoginForm')->name('login');
});

I registered that middleware to my Kernel. It is working as intended user can't logged in but I can't seem to pass that session variable so that I can send it to my view.