People use the words "double spending" as a generic term when they don't understand the details of what they are discussing (or they believe that the person that they are communicating with won't understand the details).
True double spending (where the exact same unspent outputs are spent in multiple transactions AND more than one of those transactions are in the current blockchain) is impossible with bitcoin. It has never occurred.
Generally, when most people talk about double spending, they are talking about a situation where an attacker creates a transaction that pays a vicitm and gets their victim to see the unconfirmed transaction in the victim's wallet. Then the attacker creates a different transaction paying the attacker that spends the exact same unspent outputs, and gets a miner to confirm the second transaction (the one paying the attacker). If the attacker can convince the victim to give them something of value prior to the victim noticing that the second transaction exists and is confirmed, then the attacker has successfully stolen the "thing of value" from the victim.
When blockchain.info (and perhaps some other block explorers) talk about double spending, they are talking about a situation that is quite normal and where frequently nothing was lost or stolen. Blockchain.info even says so right at the top of their "double spend" page:
https://blockchain.info/double-spendsDouble spends on this page may be unintentional. In the event that a double spend is maliciously crafted being listed on this page is no indication that it was successful or any merchant or user lost money as a result.
In that situation blockchain.info is just saying that the user sent a transaction that never got confirmed (perhaps because they forgot to include a transaction fee). Eventually, blockchain.info stopped broadcasting the transaction and made the funds available for the user to re-send. When the user re-sent the transaction, it generated a new transactionID, but used the same unspent outputs from the first attempt. Therefore, since the same unspent outputs were seen in more than one transaction attempt, blockchain.info calls it a "double spend".
