I don't think anyone intentionally upvotes the same past Twice. It can happen by accident. In my case, these accidents happened a few times.
Maybe, he is using different dapps like CTPtalk, leofinance, peakd to upvote and not realising that he already voted.
Like you mentioned, It might be people unware of it as the use differnt Hive front ends or different community.
Posted Using LeoFinance Beta