I'm always curious how does the HN submission acceptance algorithm work? I sometimes get redirected to another submission, while sometimes (like in this case) there are multiple submissions with the identical URL.

Shouldn't all same-URL submissions be considered as one, at least in a certain time period?

They're considered as one for the first 8 hours. After that, the repost only counts as a dupe if an earlier post has had significant attention in the last year or so. That's because we want good submissions to get multiple shots at getting attention.