If two different public keys had the same Base 58 hash, what would happen?
Could one key spend money in the other account?
What would happen to the monies received by the shared base58 hash?
Then both of the people could spend the same coins if they were sent to their Address, rather than public key. If they instead received the money to the public keys, they couldn't spend eachother's coins.
All in all, the most important thing that would happen is that they would show RIPEMD-160 collision, which would most likely be the first one ever. A pretty improbable sight.