I some how manage to accidentally try and upvote the same thing twice, not 100% sure how - I might have had the same post open in 2 tabs - but when I click upvote again, I received this error:
I believe that there is a validation not allowing a user to upvote more than once, so it is very likely that you did exactly what you said, had something open in 2 tabs, upvoted once, then upvoted again on the second tab which gave this error.
Anyhow, this is not a topic that should be in the 'Ideas' section, so moving this to General.