Just a quick data point: Due to personal reasons (lack of time, motivation, etc.), I decided to leave both reservations as they were without canceling the economy fare, waiting to hear from A3. Surprisingly, I received no communication—no emails or calls—and both reservations remained intact. After the flight date passed, I contacted A3 to request a refund for the taxes, and they promptly processed it.

Now, this isn’t to suggest that making duplicate reservations is a good practice, but in this instance, it seems they either didn’t notice or didn’t mind. (One reservation was made through a travel agency, while the other was booked directly with A3.) Overall, I agree that duplicate reservations typically wouldn’t remain valid for long.