Sent emails saved twice in Exchange Shared Mailbox Sent Items
Office 365 Exchange user has full access to a shared mailbox that is auto-mapped to their Outlook. The Shared mailbox has "Sent Items: Copied to Mailbox" option enabled, which should normally save a copy of sent emails to user's Sent Items, and also to the Shared Mailbox Sent Items.
This, however, doesn't work as expected and all emails sent as Shared Mailbox are saved twice into Shared Mailbox Sent Items and not saved at all in the user's Sent Items. This seems to be PC/Outlook related and does not happen in Webmail, to other users, or even to the affected user on another PC.