Exchange Online duplicate mailbox

How to resolve duplicate mailboxes in a Hybrid Exchange scenario?

It often happens that companies set up Office365 and then use it alongside the actual Exchange OnPremise infrastructure. For testing purposes or even partially productive. But when it comes to the hybrid scenario and the migration of the local mailboxes, the nasty surprise awaits. The affected user cannot be migrated because a mailbox already exists in the cloud for the same user. It is even more annoying if there is already data in the cloud mailbox. The only workaround here is usually the .pst export.

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