Symptoms
A Microsoft customer wants to buy Office 365 CSP licenses from more than one reseller. Both resellers are registered in OSA. The customer is successfully created under the first reseller. When the second reseller tries to "link" existing Microsoft account to a new customer account using the "Use Existing Microsoft Account" option, he receives an error stating that *.onmicrosoft domain already exists.
Cause
Such scenario is not currently supported in the Office 365 APS package.
Resolution
A feature request with ID POA-106263 is submitted regarding this issue.