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Normally steemit.com says "You already voted in a similar way" when I try to upvote a post that the website "forgot" about. This is the only time I caught it let me actually upvote a post that my browser isn't aware I already upvoted.

That's why I think it's a user interface issue with steemit.com. I have voted twice on the same post before by mistake but using other interfaces other than steemit.com

Ah okay, gotcha. Thanks!