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Many online forums resolve the problem you mention by showing your vote. Disqus, for example, simply shows the up arrow button in a blue highlight if you've already upvoted the comment.

If you're using Chrome I can only strongly recommend the HN Enhancement Suite which takes care of this issue: https://github.com/etcet/HNES

You could tell from which arrow got disabled by your vote.

But both arrows disappear when you vote.

[edit] Or are you suggesting a way in which the site could be modified to reflect your vote? In that case yeah, I think that would work.

Both arrows disappear when I vote (Chrome)