
The "Duplicate event" right-click menu item is only available in the web mail (OWA) and the "new" Outlook client. The good old "classic" Outlook client does not have this. To create a copy of an appointment/event therein, you can select the appointment, press CTRL+C to copy it, then CTRL+V to paste. Alternatively, hold the CTRL key and drag the appointment with your mouse to a new slot, this will also create a duplicate.