NewPipe developers are aware of the problem and are working on a fix.

In the case of Tubular (a NewPipe fork), the issue is getting code 403 (Forbidden), which prevents playing the video. It occurred sporadically while playing videos for some time but could always be fixed with a restart. Now this occurs at the very start while trying to play a video for all people at the same time. I’m strongly suspecting that this is a move aimed specifically at 3rd party players.

To detect two keystrokes closely followed, you can use EventListeners for the onkeyup/onkeydown event. In combination with a variable that determines if the key was already pressed within the last 100 ms and a setTimeout call that resets that variable after 100 ms, it should be fairly easy to accomplish.