The only way they could remove duplicates would be if the stored the IP address. Without the cookie they would have to store it with the vote. So if you have a dynamic IP address the next time you log on it is likely to be different.
However, it is technically possible to have many people assigned the same IP over time so different people could vote but if the remove duplicates based on IP address only (unlikely though), that too would make the voting inaccurate.
