Mailbox migrated to exchange online remaion on premise

I migrated a mailbox from Exchange 2013 to Exchange Online, but a copy of the mailbox still remains on Exchange 2013 server. All settings in Office 365 show the mailbox and user are ok. User can access mailbox via Exchange Online WebApp but not via Outlook 2010 or 2013 client. There are no directory synchronization errors that I can see.

I do not want to delete the mailbox because it is live.

Any advice on how I can solve this? Thank you
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Vasil Michev (MVP)Commented:
What kind of migration did you use? Doesn't look to me like a remote move.

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cyberleo2000Author Commented:
Mailbox is created in Exchange 2013 server
Run Exchange Admin Center
Find mailbox
Click Move to Exchange Online link and follow migration steps
Process completes with no errors, but mailbox now exists both in O365 and on-prem.

It does not always happen though. Most users migrate this way with no problem.
cyberleo2000Author Commented:
I've requested that this question be deleted for the following reason:

no answers
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