The issue with this is that it would be very misleading as you call exit on already completed tasks and then leaving the status as completed again. I would vote more for protecting the abort operation to not fail on such situation, meaning if the task is in different state than expected by exit command it should issue warning instead of failing on that.
Moreover there must be something wrong when you have task completed but the work item is not completed and thus process instance abort aborts the work item as well. That indicates that your process instance was not notified about task completion and thus remains in active state instead of being completed.
HTH