I will say in my little knowledge that the odds of guessing someone's private key is 0.0001 which means likely impossible, lets take an example using your mobile phone number. You might find phone number that looks exactly like yours but there is always a difference so if you put that to Private key just the same way mobile sim card creators don't allow numbers be the same that's exactly how private keys don't get to be the same. You might argue with my example that someone can guess another person's numbers, it's possible to guess a mobile phone number but for private keys the numbers are usually very long that makes it completely different from each other.