I arrived at this question when a question I answered got migrated, prompting curiosity about the possibility of double-voting.
The answers, including Oded's answer to a duplicate question, say that it is an accepted anomaly:
This is a known issue, but for such an edge case, we are OK living with it.
One issue which hasn't been addressed as yet1 is that people can sign up to different communities with different credentials. This is different from creating sock puppets on a single site (perhaps we could call them gloves - things you wear to handle different situations). It might be troublesome to maintain multiple gloves, but no one's going to stop them from doing so.
Going to the nth degree to preserve one vote per user would mean that the programmers must address the issue of identifying gloves. I'd expect that gloves require some human input to identify - and then, only imperfectly. If two users on different communities were incorrectly identified as gloves of each other, the downside would be that one of them might be prevented from exercising their vote. To those for whom votes matter at all, that's probably untenable on Stack Exchange.
1 Except possibly the waffles fan perbert as part of his wide-ranging "things that would need to be done".