how can you see whether it is the same person voting up the posts?
You can't, but if you see that a bunch of your post were each voted on once in a short period of time, that's a strong hint that a single person is going through your profile voting on all your old posts. There's nothing wrong with that, but if (for example) the votes are in very quick succession (indicating that the person isn't reading the posts) and if the user votes only on your posts and not anybody else's, then the software may decide that some kind of sock-puppetry or other abuse of the voting system is going on.
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