Since I could have owned this address prior to this thread, I could easily be lying.
The only way to prove an actually collision is by showing two different private keys that lead to the same address. Since there are many more private keys than addresses, it's much more likely to find a collision on a different key than on the same key.
(correct me if I'm wrong, but this is how I remember it)
So then the only way is for the owner of that address to come forward and have an alternative private key for this address? If they both signed an identical message would it result in different signatures?