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

Two ideas not mentioned yet are:

• Asking for the user's email address and emailing them a verification link
• Using a captcha

Obviously the former can be circumvented with disposable email addresses and so on, but gives you an audit trail, and provides a significant hurdle to casual/bot vote-stuffing. A good captcha likewise severely limits vote-stuffing, but with all the usual caveats surrounding their use.