I was checking my Movies & TV account and the posts I had downvoted when I noticed that one question appeared twice. At first I thought that it was a downvote on both the question and an answer but when expanding the information, it told me that both were on the question. I took a screenshot of the votes tab in my profile page there:
This seems to be a bug in the system. Should it be fixed?
This happens when you have two accounts that get merged together, and both of them had voted on the same post. The merge process only deletes votes that would now be to yourself, but does not look at any that would now be duplicated.
We've inquired internally about duplicate votes being deleted as well, but it wasn't high priority because users ending up with duplicate votes like this is actually quite rare. I just ran a check for duplicate votes on Stack Overflow, and this kind of error affects a whole 8,741 posts on the site, and that still includes other oddities in the past that just never got corrected (not all are from merges).
I'd like to say that we'll fix this at some point in the future, but who knows how far in the future.
Update: This is now getting fixed. See Upcoming cleanup of duplicated votes
