What is the duality of body and external name-and-form?
What is the difference between the two? What is the relationship between the two?
From the Balapandita Sutta (SN 12.19) (translated by Bhikkhu Sujato):
“Mendicants, for a fool hindered by ignorance and fettered by craving, this body has been produced.
“Avijjānīvaraṇassa, bhikkhave, bālassa taṇhāya sampayuttassa evamayaṃ kāyo samudāgato.
So there is the duality of this body and external name and form. Contact depends on this duality. When contacted through one or other of the six sense fields, the fool experiences pleasure and pain.
Iti ayañceva kāyo bahiddhā ca nāmarūpaṃ, itthetaṃ dvayaṃ, dvayaṃ paṭicca phasso saḷevāyatanāni, yehi phuṭṭho bālo sukhadukkhaṃ paṭisaṃvedayati etesaṃ vā aññatarena.
The same excerpt translated here by Bhikkhu Bodhi:
“Bhikkhus, for the fool, hindered by ignorance and fettered by craving, this body has thereby originated. So there is this body and external name-and-form: thus this dyad. Dependent on the dyad there is contact. There are just six sense bases, contacted through which—or through a certain one among them—the fool experiences pleasure and pain.
Also, as reference, from SN 12.2 (trans. Bodhi):
“And what, bhikkhus, is name-and-form? Feeling, perception, volition, contact, attention: this is called name. The four great elements and the form derived from the four great elements: this is called form. Thus this name and this form are together called name-and-form.
Also, please note that the term "kāya" has been used in the sense of physical body for example in SN 22.56 (although I know that it can be used to mean group or collection, when combined with other things):
eye, ear, nose, tongue, body, and mind consciousness.
cakkhuviññāṇaṃ, sotaviññāṇaṃ,ghānaviññāṇaṃ, jivhāviññāṇaṃ, kāyaviññāṇaṃ, manoviññāṇaṃ.