When user isn't logged in then duplicate voted are determined by user's IP, if your computers have the same public IP then you can't vote second time, to vote again you need to change public IP.

Thanks Alex️ for the follow up comment.
That is what we had expected. But unfortunately that is not the case.
As noted in the preceding comments in this discussion thread, we have tested it on different (totally unrelated) computers with different IP addresses, with visitors from different countries and even from different continent and it does not work.

LeonidS has also tested it separately and the same result.