danny hamilton is half right on this one.
there can be more then 1 private key to a single bitcoin address.. but it is most definetly practically impossible to find another key that hashes to the same address.
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Franky is correct, my answer was based on the assumption that you wanted a practical answer to your question and not a technical one.
Technically each bitcoin address has on average 7.9 X 10
28 different private keys that will all result in the exact same address.
Unfortunately, with nearly 1.158 X 10
77 different possible private keys to check, you'd have a 1 in 1.46 X 10
48 chance of any private key that you check resulting in the same address. If each of the 7 billion people on the earth had the time and resources to ALL check a billion possible private keys per second, it would take about 6.62 X 10
22 years to check 1.46 X 10
48 private keys.