Thanks for the advice, Daniel, but when I create a new ensemble with the choir, I get « Alto III » adding up to the already in place « Alto I » (choir) and « Alto II » (viola). I also tried at first to create another section player but getting the same result. I avoided to create another solo player as I want to get the full advantage of the divisi.
The project is made of different pieces brought together to form a collection of music for a choir show. Therefore, I am a bit aware that part of the problem was the importation of the songs into the main project file. I realized pretty soon I had to change the setup of some players from solo player to section player. Up to now it is fine except for this Alto confusion. By the way, I have no regret switching to Dorico!