It's generally done on the first day of a story breaking when people will be reposting the same story from different sources. It's really up to the submitter to use Algolia to check but most don't bother.
In their defense, sometimes the same story has a couple different headlines depending on which site you're posting the story from. Also, sometimes the exact same story gets posted multiple times within seconds of each other. Maybe in that second scenario the HN backend could provide a means of notifying the submitter that this exact link has already been posted?