The answer is that you cannot be sure of anything. Anecdotal evidence of what happened when won't help you here.

You need to plan on one of the following:
1. It works and you fly on one of your selected flights.
2. DL contacts you and requires you to pick one ticket.
3. DL auto cancels one of the tickets and you either find out or not.
4. DL cancels both tickets and you either find out or not.

With full flights, part of the RM/IM effort is to weed out fraudulent tickets to avoid inventory waste and unneccessary VDB.