This is a long-standing known issue with Microsoft/Outlook accounts — Outlook’s server automatically saves sent messages to Sent, but often does it very slowly. Mailspring thinks the server will not save a copy, and saves its own to ensure the sent message is not lost, resulting in duplicates.
Workaround: Go to outlook.live.com (or Outlook on the web for work accounts) > Settings > Mail > Compose and reply > disable “Save copies of messages in the Sent Items folder”. This lets Mailspring manage the sent copy exclusively.
I’ll see if we can include a broader fix for this in an upcoming relase.