Using New Outlook on Windows for personal email, calendar, and contact management
Hello, sorry for the late response.
That's odd. I was thinking that if it happens afterward, you may have the "Undo send" setting enabled. This setting makes it so that every email you send goes into the Drafts folder, then it automatically sends after a few seconds. The purpose of it is to give you a chance to edit it or delete it before it actually goes to the recipient. But it should only show up in the Drafts folder first and then only in the Sent folder, so if you're seeing a copy in both folders at the same time and it stays in the Drafts folder, then it's probably not because of that setting. If you still want to at least check that it's disabled, though, you can find it on the web browser version of Outlook under View > View Settings > Mail > Compose and reply > Undo send. I don't believe this setting is available on the desktop app.
This was the culprit for one of my clients. Thank you Angelina Harper1.