[PHP] question about preventing polling for more than once

[肖晗]

I noticed that some websites such as
polldaddy<http://polldaddy.com/account/home.php> has
fascinating poll service. And I am just curious about how it prevents user
from polling the same poll for more than once. Or more accurately, I am a
dynamic IP user. And I found that if I have polled once for a certain poll
and after some time, I changed my IP when I got connected to the Internet
again, I cannot poll the previous one for the second time. So I am confused
how polldaddy <http://polldaddy.com/account/home.php> can figure out that I
have polled even if I had changed my IP. Is there any way to achieve that?

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Thanks in advance.