Apple Mail showing duplicate sent emails

Whenever I send an email on my MacBook Pro it shows as being sent twice. This doesn't happen when I email from my phone. This only happens with my Outlook account too, my Gmail is fine. And it also doesn't happen when I send an email using the outlook app or the web browser, just when I use apple mail. And apparently, receivers only get one email, so it's not actually sending two, just showing two.

Any ideas?