Is it the same as “need”? What meaning does “duly” add to “require”?
EDIT: Adding example I found online from the 1957 Penal Code of the Empire of Ethiopia:
Art. 326. Capitulation.
(a) in battle or in the face of the
enemy lays down his arms, despatch a
bearer with a flag of truce, hauls
down his flag, or surrenders with his
men without having done everything
that the exigencies of military duly
require of him; or […]
While duly may be pleonastic in combination with require, it is probably not so here.
Military service may require various actions of you, as your superior or the rules make demands on you. In most circumstances, you must obey your superiors and the rules must be followed. However, there may be exceptional circumstances, in which a drunk superior officer orders you to shoot an entire village full of innocent people. Or a situation may arise that the rules could not have foreseen. In that case, it could be said that military service unduly requires you to carry out a ruinous action or a war crime. You are not obliged to carry out such an action and will not incur punishment if you don’t.
Even so, it could be said that the overlap with require is large enough to leave out duly. If your superior orders you to murder innocent people, that is not something military service requires of you, but rather the command of a nutcase. Similarly, the rules will usually be formulated broadly enough to allow you to break lesser rules in case of an emergency at your own discretion if you thereby uphold a greater principle. The word duly was probably thrown in as a general safe-guard without much thought.