This doesn't mean the coins have been spent twice, though. I can create thousands of different transactions which all spend the same outputs, but only one of them will ever confirm.

Really, the entire term "double spend" is a misnomer. Even if a transaction is confirmed, and then a 51% attack rolls that transaction back and sends those coins to a different address, then from a blockchain point of view they have still only been spent once since the previous transaction no longer exists in the main chain.