This doesn't appear to be the same question as: Was there a brief system glitch which caused some questions to be posted twice? or New question asking form sometimes leads to accidental double posts of questions though it's certainly possible that the cause is the same.
Thanks for raising this. I've reproduced this and confirmed with the SEDE work you've all done (thank you!) that this is still an issue. I've put this onto our backlog to address, but it may take some time before we get to it based on where our roadmap priorities are. Adding status-deferred for now, and will have the team update here once we get at this again.
Ultimately we might want to handle duplicate votes of all types, but those have a higher potential for edge cases (for example, how far would we want to or need to go to reverse any effects that those votes had - deleted posts, post owner's reputation, etc), so for now, we're going with the two simplest cases - bookmarks and follows.
Future user merges will take care of duplicate bookmarks and follows by soft-deleting the ones coming in from the merged user, and I've also removed the duplicate bookmark for this specific user.
Thanks for the report!