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Hoping you know about Application (Client) ID for every application we create in aad.portal.azure.com site. This is unique for every Application we create under every email account, right? Let's consider we got a APP ID "APP_ID_1" for a new app we created under "Email1@microsoft.com". Is there is any chance where this app id "APP_ID_1" will be created for a new app we create under "Email2@microsoft.com" (Same domain email account). Is this possible? Can we use the APP_ID_1 for email2 authentication?
I'm not certain that I understand your question. If you are asking whether the Application (client) ID for each application registered with Azure AD is unique, then the answer is yes. It is unique regardless of the user that created the app or which tenant it was created in. In other words, there will never be two app registrations with the same Application (client) ID.
Thanks for your input. Can two completely different email accounts under same domain or different domain have Same Application (Client) ID?
No - never. The Application (Client) ID is always unique.