What is the duality of body and external name-and-form?
This is just an elaboration of D.O. Everything is clear and makes sense.

Delineation of feeling depends on the notion of contact. 

Delineation of contact depends on separating totality into "internal" and "external".

Six sense doors are the subjective boundary across which the contact is made. 

On this side of the boundary are upadana-skandhas - stuff we appropriate as I/mine.

On the other side of the boundary are the namarupas, the mind-made delineated entities that we designate as "external".

In each pair of DO, it's always a notion and its counterpart serving as foundation.