Ask HN: What causes 'Sorry, that story was previously submitted.'?

I sometime make HN submissions. If the URL has already been submitted by another user, I am taken to the existing submission page, and I think the submission might also get an automatic upvote from me.

Today, I tried to submit a story and got a message:

"Sorry, that story was previously submitted."

I couldn't find any previous submission via hn.algolia.com, and it seems a few other folks have asked about this in the past, without any answer:

https://news.ycombinator.com/item?id=12104077

https://news.ycombinator.com/item?id=15445090

https://news.ycombinator.com/item?id=16404588

I hadn't previously submitted the same URL or any URL from the same domain.

The URL returned after submitting the form included 'deaddupe', which offers a clue, but it would be great to know for sure what causes this.