What is a reliable method to record votes from anonymous users, without allowing duplicates

Question

First of all, I searched as best I could and read all SO questions that seem relevant, but nothing specifically answered this. This is not a duplicate, afaik.

Obviously

Answer 1

If you're not looking at authenticating voters, then you're going to be getting some duplicate votes no matter what you use. I'd use a cookie, and have done with it for the anonymous users.

UserVoice allows both anonymous voting and voting when logged in, but then allows the admin to filter out anonymous votes - a nice solution to this problem.

Answer 2

I'd collect as much data about the session as possible without asking any questions directly (browser, OS, installed plugins, all with versions numbers, IP address etc) and hash it.

Record the hash and increment a counter if you want multiple votes to be allowed. Include a timestamp (daily, hourly etc) in the salt to make votes time sensitive, say 5 votes per day.