Why task is duplicated?
I created Task_1, for example, on September 20 and Subtask_1.1 on the same date. Then I move on to the 7-days schedule and see on September 20:
Thus Subtask_1.1 is duplicated. I think it is wrong.
Explain this point, please.
This is intended behavior. Whenever you search for something, whether it's a label or a date, Todoist will display every single task that meets the search criteria. In this case - every task that is due today and it will show all tasks on the same level even if they're sub-tasks so that you won't miss an important sub-task which would normally be hidden and you'd have to look for it by expanding every parent task to check if it's there.
In addition to that, if the task that meets the search criteria is a parent task, we add the arrow on the left side allowing you to expand its children to see what else needs to be done for this parent task.
That said, if both the parent and the sub-task meet the same search criteria then you'll see the sub-task displayed separately, but then when you expand the parent task, you'll also see it for reference.
The problem with changing this behavior is that any other options would risk missing either tasks or their context. If we wouldn't display sub-tasks separately, then in order to find them, we would have to show you parent tasks on your today list which may not even be due today, but you'd have to find the one sub-task that's due today after expanding them. On the contrary - if we would not show sub-tasks for reference and just separately, you would have to go to a project and check if the task you're about to complete doesn't have any important sub-tasks.
Therefore, the current solution allows you both - to see all tasks due today and then to quickly check if and what sub-tasks a parent task has.
OK, I understand the idea. Thanks for your clarification!