There's nothing you can do online to prevent one person to vote multiple times.
When you do a HTTP request, the website knows:
- Your IP address. This can be spoofed easily, through the use of proxies or VPNs.
- Some HTTP headers that are sent by your browser. They can also be spoofed.
For example, Tor is a browser which helps people to protect their anonymity online, by providing protections enabled by default against most browser-fingerprint vectors.
A captcha won't prevent a user to vote twice. It may prevent a computer to automatically vote, but the user is very likely to be able to pass the captcha twice if it was ever able to do once.
Solutions exist if you ask the user to give some information about its physical identity, but they're not practical for online polls.