Hi @anon71559768,
Here is the key to the answer to your question:
What you’re seeing is the normal Asana behavior of subtasks when they are attached to a project.
By default, subtasks are not attached to any project and so they don’t show up twice in the project list view. When you assign them to the project, they show up twice as you’re seeing.
Note that those top-level tasks are not duplicates of the subtasks; they are literally the same tasks, just being displayed in two places.
If you don’t need the subtasks attached to the project, you can remove the assignment to that project and the top-level display of them will go away.
A workaround that some recommend is to make a section called something like “Subtask Copies - Ignore”, put it as the bottom section in the list, drag the top-level versions of the subtasks into that section, and then collapse the section. That’s the closest you’ll get to actually hiding those top-level things!