The sources I've found don't make a clear statement of whether Aaru was formally considered part of the Duat or distinct from it. I'm trying to understand if the deceased pass through the Duat and then move on to Aaru, or if they remain in the underworld of which Aaru is a part.
There is no uniform answer to this, since the concept of Duat varies between different textual sources. What is known of the Duat derives principally from funerary texts such as the Book of Gates, the Book of Caverns, the Coffin Texts, the Amduat, and the Book of the Dead. Each of these documents fulfilled a different purpose and give a different conception of the Duat, and different texts are inconsistent with one another. Surviving texts differ in age and origin, and there likely was never a single uniform conception of the Duat, as is the case of many theological concepts in ancient Egypt.
The Amduat contains a "map" of the Duat:
The duat in this map is divided into regions, largely corresponding to an hour our nightly travel of Ra's sun-barge:
In hour 1 the sun god enters the western horizon (akhet) which is a transition between day and night.
In hours 2 and 3 he passes through an abundant watery world called 'Wernes' and the 'Waters of Osiris'.
In hour 4 he reaches Imhet the difficult sandy realm of Seker, the underworld hawk deity, where he encounters dark zig zag pathways which he has to negotiate, being dragged on a snake-boat.
In hour 5 he discovers the tomb of Osiris which is an enclosure beneath which is hidden a lake of fire, the tomb is covered by a pyramid like mound (identified with the goddess Isis) and on top of which Isis and Nephthys have alighted in the form of two kites (birds of prey).
In the sixth hour the most significant event in the underworld occurs. The ba (soul) of Ra unites with his own body, or alternatively with the ba of Osiris within the circle formed by the mehen serpent. This event is the point at which the sun begins its regeneration; it is a moment of great significance, but also danger.
In hour 7 the adversary Apep (Apophis) lies in wait and has to be subdued in chains by the magic of Isis and Ser, and the strength of Serqet, who is assisted by the god Her-Tesu-F.
In hour 8 the sun god opens the doors of the tomb and Horus calls upon a monstrous serpent with the unquenchable fire to destroy the enemies of his father, Osiris, by burning their corpses and cooking their souls.
In hour 9 they leave the sandy island of Seker by rowing vigorously back into the waters.
In hour 10 the regeneration process continues through immersion in the waters.
In hour 11 the god's eyes (a symbol for his health and well being) are fully regenerated.
In hour 12 he enters the eastern horizon ready to rise again as the new day's sun.
Once the deceased finished their journey through the underworld, they arrived at the Hall of Maat. Here they would undergo the Weighting of the Heart ceremony where their purity would be the determining factor in whether they would be allowed to enter the Kingdom of Osiris. So in the Amduat the Aaru is not considered part of the Duat.