If you’re booked on UA 4567 leaving SHR at 14:00 Tuesday and you book onto UA4565 leaving SHR at 5:00 Wednesday, UA can and will consider this a duplicate booking.

You’re also doing a disservice to everyone else by messing up the inventory numbers, making it more challenging for any of them to get re-accommodated. And, of course, if you end up actually having to use the last-minute booking you made, it might be much more expensive than just allowing UA to rebook you.