The blockchain shows coin going into MtGox, and coin coming out to MtGox... not which accounts within MtGox, holds what.
As far as the blockchain is concerned, all that coin, belongs to addresses held by MtGox.
When you deposit coin to MtGox, once your MtGox account is credited, the coin that arrived at your deposit address is moved to MtGox's wallet. (the deposit address is only used so MtGox knows who gets credited for what). Once in MtGox's operating wallet, that coin may be used for a withdrawal, or may be moved to cold storage if the operating wallet contains too much. When those moves happen, that coin is not recognizeable as yours anymore, and even the amounts of the pieces have changed.
What you think of as "yours", is no longer "yours"... and no longer exists.
=squeak=
edit: also, you are getting your terminology wrong... a double-spend, is different than MtGox having sent coin, twice... a double-spend involves two transactions trying to reference the same piece of coin as inputs to the transaction. MtGox's problem, had more to do with losing track of what it was sending, and sending more, without knowing what it sent earlier, did go through, so MtGox sent more than one valid transaction, where a double-spend, only one transaction would have succeeded.
So, those who saw that they were paid twice or more for a withdrawal exploited that. Boy if someone had gotten one of those double transactions for a single withdrawal request he'd argue loudly it can't be fixed, give up trying to fix it.