There are several options:
1) People from a country with a DTA, don't have to declare at all.
2) People from a country with a DTA, do have to declare but tic a box "not applicable because of DTA".
3) People from a country with a DTA, do have to declare but can deduct tax being paid in the other country. This would need a change to the tax form.
They can't deduct any tax beforehand as they don't know any deductibles involved. So this would mean that people might have to pay a considerable amount of tax after their tax filling has been audited.