NTFS supports Reparse Points -- MS's name for hard links -- so it's possible to have two paths refer to the same file. There is a function you can call to get the GUID of the Reparse Point. Judging by the name, this should be unique across all devices and machines.

On a given FAT device, the path is the unique identifier (once "." and ".." are properly removed). However, it is tricky to determine whether two devices are the same (in part or in whole). Given

net use z: \\127.0.0.1\c$
net use y: \\127.0.0.1\share_c
net use x: \\10.0.0.5\c$
net use w: \\tribble05\c$
subst v: c:\

net use u: \\127.0.0.1\c$\dir
net use t: \\127.0.0.1\share_dir
subst s: c:\dir
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the following could all refer to the same file:

c:\dir\file

z:\dir\file
y:\dir\file
w:\dir\file
v:\dir\file
x:\dir\file

u:\file
t:\file
s:\file
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It's even trickier once virtual drivers and shares on non-Windows machine come into play.