If you use Google's G Suite email service with Microsoft's Outlook email client using the IMAP protocol you may experience doubling of emails in your 'Sent Items' folder. This behaviour is due to the default nature of Outlook to store your sent email in Sent Items, however simultaneously G Suite saves a copy in there too! What you end up with is your sent items being saved twice.
To resolve this and restore normality in your mailbox, you will need to turn off the saving of copies of sent items. This is an Outlook setting and can be found in your email account settings.
1. Click 'File'
2. Click 'Account Settings' (a) then 'Account Settings...' (b)
3. Click on the affected email account (a) and then 'Change...' (b)
4. Click 'More Settings ...'
5. Click 'Advanced' tab
6. Tick the checkbox 'Do not save copies of sent items'
7. Click 'OK' then proceed to close the windows behind using 'Next', 'Close' or 'Finish'
[Repeat the process for any other IMAP accounts that have this issue.]
You will no longer see your sent items doubling up.