I think it's wrong for the CM to have cancelled them!
Now, I could understand double booking being wrong in the case of someone with multiple double bookings - really, make up your mind, decided where you're going to be and narrow down your dining choices.
BUT... I really see nothing wrong with being double booked on one or two ADRs.
I know for us.... we just had Nov 30th double booked at Ohanas and Crystal Palace. Certainly we wouldn't keep both of them and would cancel one... but was waiting for one of those to open up on another day. So if earlier in our trip, for instance, Ohana opened up, great, would cancel it on the 30th - or instead if CP opened up earlier day, would cancel that instead. Just matter of holding onto both till one of them opened up another day. Just a matter of keep checking online till something opened up to then cancel the double booking.
That's just an example of how I've been double booked before. Upcoming trip we have 21 ADRs (9 days of deluxe dining). Out of that, just one double booking as described above. Sorry, I see nothing wrong with that.
I DID earlier today find another Ohana opening on another day - not truly the day I want it, but that's okay, I took it. So did end up cancelling my double booking on the 30th now. Just get so worried about stuff getting cancelled.
Now... someone who has let's say Thanksgiving dinner booked at 3 restaurants, because perhaps they don't know what they'll be in the mood for that day.... or multiple days where they can't decide what park they'll be in so have ADRs booked at a few places.... or having dinner ADRs booked same day at different times, thinking now sure what time they'll want to eat yet, will decide then... that's wrong!
I see nothign wrong with the OP being double booked for that one meal, stating that they were still deciding and would be cancelling the other one.
I also see nothing wrong with the CM pointing it out - could have been a case where the OP would have been like oh, I totally forgot to cancel one of those when I found something else, just unintentionally being double booked.
I have a friend who joined us out last trip. I told her hey, for such and such date, see if you can find us something at X time at Hollywood Studios (we arrived days earlier than they did, I had no computer access and hate calling!) She calls back later... yep, I booked Sci Fi at 8:00 and Mama Melrose at 7:30 and 50's at 8:00 - something like that... that we can all figure out what we're in the mood for that day. I said ohhh no! You can't book all of them! I think a lot of people don't realize or even think about double booking is taking away from others.