This error occurs when you double-click "Next Step" or "Next Batch." There is no way to prevent it because you have created two simultaneous upgrade requests that overlap each other and pass preventative measures at roughly the same time. It might also happen if you switch to Manual on this step, thinking it is stuck when it is actually still processing a batch (browser should indicate a "loading" state); this would have the same effect as two simultaneous requests.
If this error occurs, refresh the page and it will continue normally.