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. even if you started today making 1000 keys a second and checking if they result in the same address.. you would be having to hand your project down your lineage to your great great great great great great great great great great great great great great great great great great grandchildren, and even they would still not have got a result yet
https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addressesCollisions (lack thereof)
Since Bitcoin addresses are basically random numbers, it is possible, although extremely unlikely, for two people to independently generate the same address. This is called a collision. If this happens, then both the original owner of the address and the colliding owner could spend money sent to that address. It would not be possible for the colliding person to spend the original owner's entire wallet (or vice versa). If you were to intentionally try to make a collision, it would currently take 2^107 times longer to generate a colliding Bitcoin address than to generate a block. As long as the signing and hashing algorithms remain cryptographically strong, it will likely always be more profitable to collect generations and transaction fees than to try to create collisions.
It is more likely that the Earth is destroyed in the next 5 seconds, than that a collision occur in the next millenium.