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

I have the same problem, and here's what I am planning on doing...

Set a persistent cookie. Check the cookie to decide whether a particular vote could be cast. Additionally store some data about the vote request in the form of a combination of IP address + User Agent. And then use this value to limit the no. of votes to, say, 10 per day.

What is the best way of going about creating this hash (IP + UA String)?