From what I can gather the official Due's, both v1 and v2 pin out only 12 ADC lines.
Note 1. AD15 is not an actual pin but is connected to a temperature sensor.
Looking at the datasheet, it looks like the official pinout is wrong. The other ADC inputs are pins 20, 21 and 52. The 16th ADC channel is connected to an internal temperature sensor, not an actual pin, so the "16 analog input pins" on the product page is incorrect as well.