I’ve got a problem with iOS 6 that persists even now with new updates. Right now, I have 4 email accounts, 1 iCloud account and 3 Outlook ones. Those three are always duplicates of sent mail within the app. They cannot be found on the Sent folder and recipients receive only one mail. What is going on here between Microsoft and Apple?
I am sorry but are you still running on iOS 6 in 2019?
I sugguest to get a latest update available for your iOS device as I presume its just an old unfixed bug.
1.Go to your iOS device settings
Tap Mail, Contacts and Calendars
Under the Accounts section, select email account that is sending duplicates
Tap Delete Account
Confirm that you want to delete email from your device
Under the Accounts sections, tap Add Account
Add the Outlook account again.
Please check the link below