>>I've read that a GUID is guaranteed to be unique worldwide, but I always took that to be specifically in the context of its designed-for use. In that context the "generator" can use the registry to double-check this (at least as I imagine things). Do you check the returned GUID against existing keys to assure uniqueness?
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It's not necessary. Globally unique means unique across space and time. CoCreateGUID creates a 128 bit value, which means that there is a 1 in 281 trillion chance of ever creating that value again, anywhere, anytime.