Why did they place two resistor arrays RN1, RN2, if it was possible to place one resistor array 100k with four resistors RN1A, RN1B, RN1C, RN1D (see schematics of Arduino Due)? Connections can be made on the printed circuit board, there is enough space.
I agree, and I don't know, but this section doesn't make any sense to me:
Couldn't they just let those spare resistors unconnected?
Hi,
Not sure about the left hand array wired like that, but resistor arrays are great for quick assembly and use in small spaces on the PCB.
In your diagram, the pull up resistor array would be physically placed next to the PCB connector, and would only be 5 connections, 4 discrete resistors would be 8 connections.
Tom.. 
That is a designer decision and is probably based on what they use in production and what there production equipment is capable of inserting. Wattage may also play a part.
The power dissipation on each of the resistors is 0.1 mW, which is much lower than the permissible one. The production equipment remains the same, since instead of two arrays of resistors, you can install one exactly the same
Did they actually use arrays on the board?
I don't have a due but from images, it looks like 4 separate resistors
On the PCB (see below), they are highlighted with a separate rectangle, therefore, this is an array of resistors, and not individual resistors.
Thanks
Nope, if they were separate resistors it'd be completely useless to wire 4 more resistors that way (completely short-circuited to GND).
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