Preventing spoofed/duplicate accounts (while ensuring all legitimate accounts work) is a very difficult task -- for reasons laid out by others. In addition to trying to guard against concurrent multiple accounts, one must guard against non-concurrent multiple account usage.
The core issue isn't so much in determining where an account connected from, but being able to trust that a user only has one account -- and to this end the only "real" solution is to use a system which already provides this sort of information, such as a credit card or paypal account ;-) That is, simply prevent someone from creating a new account (although an account can have multiple aliases/profiles, but these can be trivially tracked) unless they can prove "uniqueness".
(Also consider that two people may have two different accounts on the same machine.)
Happy coding.