Hi @Cloud_Geek_82 ,
Typically, this issue occurs if the calendar that you're sharing is listed two times in the Calendar pane in Outlook.
You can try this.
Use the Outlook for Windows client to delete the calendar
In Outlook for Windows, open the Calendar pane, and then locate the calendar name that is the same as <Mailbox Owner>.
Right-click the calendar, and then select Delete Calendar.
Restart Outlook.