Subtasks duplication error

Welcome to the Asana Community Forum @Riccardo_Moro!

That is currently expected behaviour. If a recurring parent task has subtasks that are assigned with due date (no matter if marked complete already or not) then once you mark the main task complete and a new one is created the subtasks are created again too.
Have a look here and here and this.

So you might have to work something out with how you use these subtasks and rather work with tasks instead. Or try to work something out with rules (rules can now also trigger subtasks being added or new tasks being created so that might help)