Mac,
I don't agree.
To start with, there are so many different designs for single triggers in double rifles that you cannot say that what does, or does not happen with one type will be the same for all single trigger systems.

It IS possible for shooter technique to cause some single triggers to double, and feathering the trigger is a perfect way to get a DD with some single trigger types even if all the lock system is functioning correctly and safely.
Just as you need to use a suitable technique with two triggers, you also need to do the same with singles.
Some shooters will naturally use a good tech and will not even realize that they are doing it.
Others will use an inappropriate technique and will have problems, one being double discharging.

Of course it is always possible that there was a fault of some kind which caused the doubling on that particular double, but you cannot rule out shooter error with single trigger systems.