Okay, I have a Wood-Elf character that is going to become a Monk that follows the Way of Shadow. A Way of Shadow Monk gets the ability to cast the spell Darkvision (among others) using Ki. I want to know how the spell interacts with races that already have dark-vision. Does it add to the races natural dark-vision? or Does the spell simply overlap?
You would gain no additional effect.
There is nothing in the spell's description that says you gain anything but 60 feet of darkvision. It is widely accepted that spells do only what they say they do.
Darkvision PHB p230:
You touch a willing creature to grant it the ability to see in the dark. For the duration that creature has darkvision out to a range of 60 feet.
Compare that to the Goggles of Night DMG p172
If you already have darkvision, wearing the goggles increases its range by 60 feet.
If your DM wishes to give additional benefits because "its magic" that would be a house rule.
Personally, I don't really do that sort of thing because I look at it in a way that some spells were created by creatures that don't have such abilities and the base spell would not have considered a creature with that ability.
However, it is plausible that a variant would have been developed by a society consisting of those with darkvision that would extend it, but that would not be a common spell in my thinking and would have to be found while adventuring. That spell might not grant any benefit to a creature without natural darkvision.
Spell overlaps natural ability without benefit
The spell Darkvision grants a willing creature the ability to see in the dark.
For the duration that creature has darkvision out to a range of 60 feet.
If the recipient of the spell already has darkvision with a range of 60' or more, the spell provides no benefit.