Laravel Session return previous value

When I write this line of code of my controller

public function cartAdd(Request $request){

    $request->session()->put('key', 'value 1');
    $output = $request->session()->get('key');
     dd($output);
}

Then output "value 1"

But when I comment one line code which set the session value

public function cartAdd(Request $request){

  // $request->session()->put('key', 'value 1');
    $output = $request->session()->get('key');
      dd($output);
}

Then output

"this is key 1"

This is my previous session value. What's wrong with this. This time output should be "value 1"