No, Delta did not authorize twice. From what I read here, this is my take:

• May23, DL did the authorization. This posts (if approved) to your acct as a "pending(floating??) transaction". It counts against your credit line, but is NOT a billable transaction that you owe anything for. It will never show on your statement.
• May24, DL ticketed the reservation and sent a "record of charge" to your cc bank with the ticket number to prove that the purchase was completed. This posts to your cc acct as a "real" transaction. It is billable and will be on your next statement.

ANY credit card purchase where the merchant did an approval first (so practically all transactions) have these 2 transactions postings: 1) the authorization (pending) 2) the record of charge (real).
When all works ok, the pending transaction is deleted at the same time the real transaction is posted. So from your point of view, you think of it as one transaction. But behind the scenes, it's always two.
For the "auto delete" of the pending transaction to happen, the associated record of charge must match to it. Many things can cause the match to fail but most likely it's some batch job held up somewhere. It could the DL or credit card end.

Any pending transaction that doesn't get matched/deleted will eventually expire. At that time, the "hold" against your credit line for that amount will be released. The expiration time frame is variable, but think of it as 7 days +/- a day or 2 and you'd be close most of the time. As long as the pending transaction isn't pushing you up to your credit limit, it's completely harmless. As others have noted, this goes on all the time. It usually goes un-noticed, but airline purchases tend to be larger amounts thus more 'visible'.