Duplicate emails in Sent Mail folder when sending from Outlook with Gmail account

Symptom:
Duplicate emails in Sent Mail folder when sending from Outlook with Gmail account.
(Delivery just once, but store twice.)

Solution:
Go to File => Account Settings => Account Settings => (your gmail account) => Change… => More Settings… => Special => Sent Items and then check the option "Do not save copies of sent items"."

Settings of different Outlook versions:
https://www.msoutlook.info/question/prevent-duplicate-sent-items-gmail

Source:
superuser.com