See my questions below regarding the “All Tasks” checkbox; that’s what you meant by “Tasks” being checked, right?
But I think you may be confusing things re “mass deletion is still needed.” Deleted tasks are not found in the duplicated project regardless of the checkbox setting. (To be clear, the following are all different states for tasks: incomplete, completed, deleted, permanently deleted.)
@Emily_Roman, What does the first checkbox, All Tasks," at the top of the Duplicate Project dialog do? I tried it checked and unchecked and the results seem the same to me with respect to tasks in the duplicated project. Also, that checkbox is not documented at Setting Up a Project in Asana | Product Guide • Asana Product Guide
Thanks,
Larry