In the latter case, I think it might be feasible to prevent upvotes from being counted multiple times if the username is identical on different instances, since upvotes are public. Is there already a mechanism to do this?

Also, isn’t it much more common in the Fediverse than on central platforms for the same user to have multiple accounts with different usernames? This seems likely to me, if only because popular usernames may already be taken on a given instance. In this case it seems to me hardly possible to prevent double counting. I suppose this would only be possible if the different instances would log IP addresses and share this information with other instances. That doesn’t seem desirable to me at all, and probably wouldn’t be legal, at least in Europe, because of the GDPR. Are there other possibilities? Cookies?

Please excuse the maybe stupid questions - I’m new here and not very good at finding info on my own yet…