Why on Earth would you trap back button on your site? It's despicable and makes your site immediately untrustworthy.
Worse yet, it seems limited to when it's navigated to from HN specifically. Reproduced on Chrome, Dolphin, and Brave.
Ah crap, think this is a bug in the client-side routing, pushing two entries into the history. I think it has to do with the query parameter. Will fix, thanks for letting me know :)