Just tried something that occurred to me because I though I could find a large bug. It only partially worked, so I guess it's at best a small bug...

Basically, I tried opening the same node in multiple tabs of my browser prior to voting on it. Since the voting options stick around in any tab opened prior to actually voting on a node, and curious if the programming behind the scenes actually just hid them from the account that already voted after the fact to prevent multiple votes, or if it did any kind of back end checks to see if that specific account had already voted on the node, I gave it a try.

From what I could observe, my first upvote did increase the rep of the node, but my second upvote did not. I'm not sure if there is any wonky behavior involving XP for either the voter or the vote-ee, but I did notice that my number of votes left for the day decreased in both cases.

So based on that, I'm calling it a small bug, since really if someone tries to do this maybe they should lose the vote(s) to oblivion anyway (assuming that's all that happens). I figured I'd report it none the less, since sometimes I have an annoying habit of trying to "break" things like this on purpose. Trust me, I'm a video game designer's/programmer's worst nightmare, after beta testing and post release or course. :-)

UPDATE: My bad, disregard based on my response to soonix below. That's what I get for taking non-current data at face value, and just generally not thinking through what I was actually observing. I should have seen my number of votes decrease by two instead of one the second time if what I said were actually true.