I suggest to have an option, to mark a task as partialy completed and stop postponing it to another day.
When copying such a task, a user should be asked if he wants to persist the subtasks' completion as it is from the original task or not. If so, the original task will be closed and the new one will be created with some subtasks already completed. If not, a new task will be created with all the subtasks opened (as it is now).
This scenario is great for shopping, if you do not want to buy everything at once and have not planned another shop visit. For example, if you want to reuse a shopping list and you know, that you would not be able to eat it all before the food quality expires or you do not want to eat everything from the list before. You will see immediately, when you shopped last and what you bought. Maybe, consider if partialy completed tasks should count as completed for the menu link "Tasks (n) incompleted".