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).
So the question is: Could it be possible that a collision already happened in the past, we just don't know it? And even the "victims" of a collision do not know?
Or, in other words: Is there any way to find out if a collision already happened in the past?