The following is from the Bitcoin wiki:
Since Bitcoin addresses are basically random numbers, it is possible, although extremely unlikely, for two people to independently generate the same address. This is called a collision. If this happens, then both the original owner of the address and the colliding owner could spend money sent to that address. It would not be possible for the colliding person to spend the original owner's entire wallet (or vice versa).
Sorry but if you've included the next sentence in that same bitcoin-wiki article, your question would sound silly.
Since it already provided a good representation of its chance:
But because the space of possible addresses is so astronomically large it is more likely that the Earth is destroyed in the next 5 seconds, than that a collision occur in the next millenium.
And it's not even an overstatement when it comes with collision.