Microsoft Outlook 365 double some emails

I noticed that the Outlook 365 app (not on the webmail, I think) created two of the same email (if the email is sent to my Gmail account), but one email is with a reference (red marked) to the original email account (see screen dump).

Strange behaviour, and I have looked at the Gmail email account settings but could not find any "connection" to my Outlook account. 

Probably a known problem, and several people have experienced this issue, so I hope this can be solved quickly.