Earlier, I posted a comment on this question. Later, someone let me know something had changed in the question, and asked me to delete that comment. I got a notification for that comment as using @JerryDodge. So I deleted the comment. The original commenter then deleted theirs too, leaving the question commentless.
About an hour or so later, I got another notification for that other user's comment, even though it was deleted. I just assumed a fluke in the system, so I ignored it. But even about an hour later, I again get notified for the same comment - a third time. Which was deleted long ago.
This occurred even after closing the browser completely and opening it again later.
A possible reason is the fact that I had voted to close the question as duplicate of the same user asking a different variation of that question previously. I had first posted a comment advising the OP not to ask the same question twice, and the second comment was automatically created when I voted to close as duplicate. Then, the original duplicate question was deleted, thus cancelling out the close vote(s) and deleting the comment automatically created by my vote.