I have a strange issue and I cannot wrap my head around it. When our secretary goes into her Outlook 365, and selects a shared Mailbox, there she forwards the e-mail to someone in the company - the sent items will show two identical e-mails in the shared mailbox. So far I am aware of there isn't a rule on the mailbox or on the server side.
We are using hybrid exchange setup with only one exchange server on-premise and online.
Where can I start to look for the possible cause?