The only time I had 2 ADRs booked for the same time was for a couple of minutes, literally...it was back in 2008 when there were no online ADRs yet. DH and I had originally booked Ohana for dinner one night of our trip but then decided we would rather go to 1900 Park Fare instead. I called the dining line and booked 1900 Park Fare. As soon as I got the confirmation number, I asked the CM to please cancel Ohana and she did.
I agree that all ADRs should be cancelled if someone books ADRs that would overlap in time. Meaning, if someone books Le Cellier for 5:15PM and Les Chefs de France for 6PM, the system should cancel both ADRs. I don't think it is fair to others. I know many of these ADRs are hard to come by, but if people only had a choice to book one ADR for dinner and not multiple ones "just in case" then maybe some of the most popular restaurants would be easier to get for people who are really sure they want to eat there. Meaning, if you have to wait a little bit longer to make your ADR or change one, maybe something would be more likely to be available if people were unable to double-, or triple-book ADRs.
I would not mind having to give a credit card hold on ADRs at all, as long as there would be a reasonable cancellation policy. If I can cancel the same day without a charge I think that would be fair. Only because with children, you never know...they could literally become sick (although it is unlikely to happen) an hour before your ADR time. It would be kind of like a hotel cancellation policy...you know, where you can cancel up until a certain time the day of check-in (4PM or 6PM in my experience). I think people should be charged a certain amount for being a no-show and then we would see all the no-shows stop, people being able to book the ADRs they want, walk-ins not being turned away as much, and not seeing restaurants at WDW as empty as people currently report.
On the other hand, a couple of years back, a colleague of mine was booking 3 or 4 different places for dinner (because she didn't know which theme park they would be going to on a certain day), and I "yelled" at her. I said to her "you're going to cancel the other ones as SOON as you know right?" Her reply was "Why do I have to cancel? If someone didn't get it, that's their hard luck." Nice attitude, huh?!