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.

Another thing I was thinking of is this setting on the desktop app. It sends a pending email into the Drafts folder if it has been inactive for a set amount of time, but since your problem happens after you attempt to send an email, so this is probably not it either. If you still want to check it just in case, you should find it under File > Options > Mail > Save messages. Doesn't look like this setting is available on the web browser version.

I don't think either of these would be the cause of the issue you're having, but I figured I'd at least mention them in case they may be enabled but malfunctioning for one reason or another. Sorry I couldn't be of anymore help!