By default, Outlook for Mac will send emails using the ‘Reply-To’ address that is in your settings rather than letting the server end decide which to use.
This may lead to you getting copied into emails that you are sending from Outlook and seeing multiple instances of sent mail.
To correct this please update the follow :
With Outlook open, click the Tools menu and then Accounts. We need to edit the Reply To address in the Account setup window to match the below based on your email address :