Maybe somebody else created a keypair for that address? I know it is very unlikely, but it could happen. So I would spend any money in that address and not use it anymore. Just in case.
To all intents and purposes it couldn't happen, if all software is functioning correctly, any more than all the atoms in someone's underwear could just simultaneously decide to move three feet to the left. Quantum mechanically you could argue that it's possible but unlikely, but that's just sophism (or more accurately fine comedic writing).
But, of course, software can have bugs. Could there be a flaw in the random number generation which makes a collision plausible? Sure, there could be.
Still, I don't believe it, given the evidence in the blockchain.
