Is a GUID unique 100% of the time?

Is a GUID unique 100% of the time?

Will it stay unique over multiple threads?

While each generated GUID is not
guaranteed to be unique, the total
number of unique keys (2¹²⁸ or
3.4×10³⁸) 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 5×10²²
stars; every star could then have
6.8×10¹⁵ 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.

Guid guide, part one

Guid guide, part two

Guid guide, part three

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