If you're using Outlook 2016, click on 'File' (at the top left) and then on 'Account Settings' (as shown at around the 40 second mark in the video I provided you with on your other thread).
If you're using a different version of Outlook, please tell us which one! (There have been 7 editions of Microsoft Outlook over the years. They're all different!)
To view all settings.
Click "wheel" in top right that says settings with a cursor hover.
Bottom of page right pane that says "View All Outlook Settings.
All your layout controls and preferences are in there under different sub headings.
Couple of questions. Are you getting duplicate emails "sent" or just duplicate "sent items" messages? Are you using Gmail? Are you synced with another email address? What are you using, i.e. pc, tablet, laptop ?