Wow...they are going to start cancelling ADRs and then contact guests? Perhaps it should be the other way around. Or maybe they shouldn't take two bookings for the same people at the same time in the first place. I also hope there's a note section so that parties that are dining in separte places at around the same time don't fall into this double booking net and get cancelled.
There's going to be havoc at the restaurants when these people show up for dinner and told - sorry we don't have you down. Actually, what I see happening is the restaurant squeezing them in anyway so there's no uncomfortable scene at the podium.
Two years ago, my dh made some ADRs and our name was spelled wrong in the system. We didn't have the confirmation number but none of our five phone numbers showed up in the system.

We didn't cause a scene but the restauant manager got us a table even though the restaurant was booked. (We figured out the wrong spelling error at our next meal.)
But if they really want to start enforcing the no double booking rule then they are going to need to come up with a different system for recording ADRs.
Most people have multiple phone numbers they can use for reservations plus there is the hotel confirmation number.
I never double book but thanks for the heads up!!