What if someone else just so happens to randomly generate the same keypair and someone else?
As 'error' noted, it's extremely unlikely to happen. If by some chance it did happen, then any bitcoins sent to that address would appear in both users' wallets. When one of them spent the money (even partially) the entire amount of the original transaction would disappear from the other person's wallet.
Like what if someone were to setup their machine to do nothing but generate keypairs until they found one that matched an account with coins in it? Hell, while your at it, same all the keypairs you make and check them periodically for matches.
The entire science of cryptography is geared towards making sure that kind of attack is computationally infeasible. For you and me, "computationally infeasible" essentially means "impossible."
