PROBLEM
When you try to verify a domain in Microsoft Office 365, you receive one of the following error messages:
<DomainName> has already been verified for your account, or for another Microsoft Online Services account.
Can't verify the domain <DomainName> was already added to a different Office 365 account domain.onmicrosoft.com. Sign in to that account as an admin, and remove domain <DomainName>. Then come back here and try adding <DomainName> to this account again
CAUSE
This issue may occur if the domain exists in another Office 365 organization.
SOLUTION
Do one of the following, as appropriate for your situation:
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If you previously verified the domain in Office 365, you must first remove the domain from its current location. The domain can only exist in one location in Office 365.
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If you don't know what Office 365 account is currently using the custom domain or if you don't have control over the Office 365 account that's currently using the custom domain, contact Office 365 Support.
REFERENCES
For more info about how to troubleshoot domain verification issues in Office 365, see the following Microsoft Knowledge Base article:
2515404 Troubleshoot domain verification issues in Office 365
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