This issue has driven me nuts for a very, very long time, before Big Sur. I'm posting to see if others are experiencing this issue and to see if there's a fix. I have an @outlook.com email account, and I use the Apple Mail app on my MacBook Air and my iPhone as my sole account. Anytime I send an email from my Mac, the email appears twice in the "Sent" folder. I don't think it actually sends the email twice, but it appears as two separate emails. If a conversation lives on for a long thread, I see two emails sent there too when looking at the full conversation in the Archive folder. Any others experiencing this issue? Anything I can fix to resolve it?