Check whether the same email account is configured twice in "Microsoft Office Outlook".
Open Outlook > click Tools > Account Settings > click E-mail tab.
If the steps above did not work you can also try creating a new folder in your web-mail and move all the e-mails from inbox to a new backup folder. This is to check and move any damaged or corrupt emails in the server.
Now try to send a test email and check if you receive duplicate emails.