There is a well known bug in the Bitcoin protocol which enables the same coin to be transferred more than once. Every exchange knows about it and they all defend against it. Except evidently Mt. Gox did not. It’s an absolutely stunning failure. It’s a bit like a bank manager not bothering to lock the vault at night.
Each such failure like this is also an interesting study in human folly.
“There is a well known bug in the Bitcoin protocol which enables the same coin to be transferred more than once”
Gosh, don’t you just hate it when your money catches a protocol bug?
Unless you know of something new, this is a misrepresentation of an issue known as transaction malleability. Yes, exchange and wallet software developers need to be aware of it, but it does not allow the same coin to be transferred more than once.
If you know something new, we'd like the details.