This question already has an answer here:
Simple case, for comments:
- Navigate to a question page that has a link away from the current SE site.
- Upvote a comment. [Most probably there’s an issue with flagging as well.] The comment score is updated.
- Click the link. [Most probably the link is not essential and any navigation works.]
- Click the browser back button to return.
- Observe your upvote is not displayed, try to upvote once more.
- Get an error popup saying “you cannot mark a comment more than once”, observe that your upvote is still not displayed. Refresh page, observe your original upvote is correctly recorded.
Complex case, for questions and answers (could actually be a feature, or rather a somewhat confusing workaround that wasn’t applied to comments by mistake):
- Do steps 1–5 above, but for a question or answer rather than a comment.
- Observe what looks like a successful upvote. Refresh page, observe that the question score has (correctly) only been increased once even though you pressed clicked the upvote arrow two times overall.
ETA I’d argue that, even if showing the page as it was when I navigated from it is infeasible, at least the reaction to upvoting an already-upvoted comment should be the same as for upvoting an already-upvoted question or answer: either both should show an error message, or both should succeed.
[Browser: Chromium 64.0.3282.167 on Linux x86-64]