I have the same problem with both iCIoud and Outlook IMAP accounts creating duplicates in the Sent folder. I had to disable the mail setting that saved a copy in the sent folder to avoid eM client crashes when sending mail from my iCloud account.
https://forum.emclient.com/emclient/topics/application-error-loop
I am running version 7.2.36908. Even with the mail setting disabled, I still get a copy created on my Outlook account. All other accounts both IMAP and POP3 get no copies in the Sent folder once the setting is turned off.