Oh, it happens all the time. Usually we post a comment saying so (here are 2,000 of those: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...), but sometimes we don't have time or are on mobile, etc.

I've added such a comment to https://news.ycombinator.com/item?id=45155452 now, which is where it would usually go.

Nice. Idea: it would be cool if HN kept the original submission just as signpost or tombstone, that would be incredibly transparent (otherwise tracing URL modifications depends entirely on the submitter mentioning it and being honest about it). Although I admit this is a very minor feature. I know stack overflow does a vaguely similar thing with duplicate posts, where they'll usually be downvoted and closed quickly, but they often remain as 'signposts' linking to the preferred post on that topic.

Btw, thanks for all the work you do.

It happens sometimes on HN. Times I’ve seen it happen is when another article covers the events better, or are just the original article instead.