I recently upvoted a comment (first time upvoting that specific comment) on my meta site and received an error message stating "You cannot mark a comment more than once." Another mod mentioned they encountered the same thing occurring on their site.
After a bit of testing, it seems to only affect moderators when they are voting on a site they moderate.
Possibly related to this bug or its fix.
As mentioned by @Makyen,
The bug is that two POST requests are sent when the comment upvote button is clicked. Depending on the time between them and the exact servers which they hit, I'd expect to intermittently see an error.
I have also confirmed myself that 2 POST requests are sent when I upvote a comment. The first succeeds, while the second fails with the above error message.