Hi @James_Cooper2 , this is happening because the subtask Milestone ‘Deliver analytical capability’ has been added to the project, which you can see in the field ‘Projects’ with the name crossed out. If you click on the ‘x’, it will solve your issue by no longer showing it also in the list view as a parent task (note, it is technically NOT a duplicate).
Note, that this will also reflect in the List view, so you will no longer see your subtasks in the main list view.