There's an answer in old thread, my suggestion if you have a question, it's better to use google search by type "bitcointalk.org: your question", but if you don't find any answer then you can create new thread.
This would be called a "collision" and is highly unlikely.
Keys are 256 bit in length and are hashed in a 160 bit address.(2^160th power) Divide it by the world population and you have about 215,000,000,000,000,000,000,000,000,000,000,000,000 addresses per capita.(2.15 x 10^38)