A common issue that I see when migrating user mailboxes from Exchange on-premises to Exchange Online is that, somehow, a user ends up with two mailboxes. This can sometimes happen because a license is assigned to a user before Exchange Hybrid is implemented or an error during synchronization. To remedy this issue is no small feat and usually requires the removal of the cloud account, which could cause data loss depending on how many services have been consumed in Office 365.
Microsoft have now provided an easier way to remedy the above scenario.
Set-User email@example.com -PermanentlyClearPreviousMailboxInfo
The above command will remove the cloud mailbox from the account and allow for the account to be resynchronized and on-board the mailbox.
Much easier 🙂