H @Sophie_Magdalene, welcome to the Asana Community Forum! As it stands, it’s currently not possible to change the pre-selection of options when duplicating a task or project. For projects, I recommend you creating Custom Templates.
We have exciting news coming for templates and tasks this month! Stay tune and keep an eye at our announcements category! (Follow our News category to keep up to date with what’s new in Asana! 📨)