It's so unlikely that it's as good as impossible. To calculate the same address and private key as someone else is more unlikely than getting hit by lightning at 12 noon for six consecutive days, then winning the lottery on the 7th day. If you search bitcointalk you will find countless threads by people who lost their private key for their Bitcoin wallet and want to know how to get their Bitcoins back. The short answer is without their private key they can't.