I Have a Double Decker WHITE Coneflower...
and it was a plant that naturalized itself! I did buy pink double deckers and last year I had about 8 true doubles on the plants. Most of the other flowers on the DD were normal, a few had so many petals that they looked like a carnation, and some had a thicker layer of petals in the right place.
This white coneflower plant has one double decker, true to the pictures on the nursery web pages, and all the other flowers are normal, so it must just be a fluke. This year so far I only have one true double decker on my pinks.
I will post a picture later when I have camera in hand.