Duplicate Emails in Sent Folder

Hi Dalry1826,

Thanks for coming to Apple Support Communities for help today.

First, we'd suggest checking the Outlook webmail portal to see if there are duplicates found on the site. If there are, you'd definitely want to talk to Microsoft Support.

If that doesn't help explain the issue, do you have any Rules set up in Mail > Preferences > Rules for this account? It's possible there could be something duplicating the sent mail for you.

Finally, try removing the account and re-adding it. This may trigger the account to fix itself. You can se how to remove it here: Remove email accounts in Mail on Mac.

"Remove email accounts in Mail on Mac

You can remove an email account from the Mail app on your Mac at any time. When you do, the account’s email messages are removed from your Mac. Copies of messages remain on the account’s mail server and are still available (from webmail, for example). 

Important: If you’re unsure whether messages you want to keep are still on the mail server, move or copy them to a mailbox stored on your Mac (the mailbox appears in the On My Mac section in the Mail sidebar) before you remove the account in Mail.

1. In the Mail app  on your Mac, choose Mail > Preferences, then click Accounts. 
2. Select an account, then click the Remove button .

Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts System Preferences. Click the button to open Internet Accounts, then deselect the Mail checkbox for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button"

You can re-add the account by following: Add email accounts in Mail on Mac.

"If you already added an email account, you can still add more. In Mail, choose Mail > Add Account, select an account type, then enter your account information. Make sure the Mail checkbox is selected for the account.

If you’re using an account on your Mac with other apps, such as Contacts or Messages, you can also use that account with Mail. In Mail, choose Mail > Accounts to open Internet Accounts preferences, select the account on the left, then select the Mail checkbox on the right."

Let us know if the issue continues after this.

Have a fantastic day!

Hello Dalry1826,

Thanks for following up! Since the issue here is specific to one account, we agree that getting in touch with the email service provider is your next best step. Keep us posted on how it goes.

Have a good one.