<p>Is a GUID unique 100% of the time?</p>

<p>Will it stay unique over multiple threads?</p>

While each generated GUID is not
guaranteed to be unique, the total
number of unique keys (2
128
or
3.4×10
38
) is so large that the probability of the same number being
generated twice is very small. For
example, consider the observable
universe, which contains about 5×10
22
stars; every star could then have
6.8×10
15
universally unique GUIDs.
From
Wikipedia
.
These are some good articles on how a GUID is made (for .NET) and how you could get the same guid in the right situation.
https://ericlippert.com/2012/04/24/guid-guide-part-one/
https://ericlippert.com/2012/04/30/guid-guide-part-two/
https://ericlippert.com/2012/05/07/guid-guide-part-three/
​​