I was able to reproduce the issue by opening the same post in a new tab, upvoting a comment from the earlier tab, and attempting to upvote the same comment from the new tab.
While this message may seem like it can only be triggered if one tries to intentionally exploit it, it can also be triggered in certain accidental circumstances, e.g. if you navigate back to the page, your browser may have cached the prior version of the page that didn't show your upvote, and one might think "I remember upvoting this" and trying again, only to receive the error message.
At the very least, it should be made less confusing:
You cannot upvote a comment more than once
An alternative, possibly better solution would be to do the same thing that happens with post votes in this case: appear to upvote the comment and update the score, but do nothing on the backend.