When you have cast your vote, if you try doing a hard refresh, does the vote not show then? If it does not show, there's something wrong which we will definitely need to fix ASAP.
Yet it seems to work ok. My bad. I checked in different browsers, and my vote is counted only once.
However, I hope you will be able to protect this voting system from manipulation.
btw. I think it would be good to write somewhere (maybe at the bottom of the page) that some links can contain an affiliate link. some platforms (FB for example) are quite restrictive when it comes to sharing referral links, if you have made this clear, you reduce your chances of being blacklisted from their side.