If you have registered a domain in Office 365/Azure Active Directory, and after a while, you try to register the same domain in a different tenant, you would see the “verify you own this domain” screen and suffer a lot of headaches because a ticket would be required to find out where that specific domain is in the Microsoft cloud. That was a common issue a few months back. Today, I was working on my Microsoft Teams articles here at TechGenix, and when I tried to register my domain, I got an error that the domain was already registered. Still, the difference nowadays is that the verify you own this domain error message now informs you precisely where the domain is being verified.
That saves a lot of time for the IT professional. Now it is just a matter of logging on that tenant and removing the domain from there before continuing. Make sure that you spare at least a couple of hours to flush the deletion in one tenant and reuse of the domain in a different tenant.