Q:

Azure Active Directory User Object Id (Guid) uniqueness across all AAD domains

I was not able to find any info on this. Can a user from any AAD domain have the same Object Id as a user from any other domain.  Or are these unique across all domains in Azure?
EDIT:  Is there a validation process in Azure that checks if a GUID has already been used for the same type of object, such as a RID Master in Windows AD..

A:

Yes, all of the users in Azure has an unique object id no matter where the user comes from because the object id is GUID format, GUID is Globally Unique Identifier, it will generate an unique GUID for each user when add a user to Azure AD.
By the way, there could be two same object id(guid), but it's a very small probability(very very small, for example if you have 100,000,000 users in all of your AAD domains, the probability of exist two same object id is about 1/3.4×10^30, so we can ignore that).

