Steht doch in
https://help.hcltechsw.com/traveler/10.0.1/topics/iOSKnownLimitations.html:
Apple iOS/iPadOS 13.0 and later: after sending one email from the Mail app, two copies of the email appear in the Sent folder.
As of iOS/iPadOS 13.0, Apple devices add an entry to the Sent folder for any emails sent from the Mail app. When the Sent folder is synced, the server entry is added and the device does not remove the original, resulting in a duplicate.
Not fixed.