Are you using the Email Connector module? If you are, then if you have used the CreateEmailFromTemplate action to create an EmailMessage object as in the examples, then you can simply change the EmailMessage object to remove the cc. This would be a decision split to see if the To and CC attributes are equal, if they are just use a Change Object action to set the CC attribute to empty.
I hope this helps. Good luck!
From what I’ve read, the easiest approach would be to customize the Send Mail microflow so that it checks both the “To” and “CC” lists. If a duplicate email address is found in both, it removes one of them, ensuring the email is sent only once.
Make sure to save this customization in a separate module—ideally in a dedicated CustomEmail module—so your changes aren’t lost when you update the Marketplace module in the future.