For adding followers in CC as recipient you need to inherit get_mail_values() method of mail.compose.message object and Update below values:

mail_values = {

                'subject': self.subject,

                'body': self.body or '',

                'parent_id': self.parent_id and self.parent_id.id,

                'partner_ids': [partner.id for partner in self.partner_ids],

                'attachment_ids': [attach.id for attach in self.attachment_ids],

                'author_id': self.author_id.id,

                'email_from': self.email_from,

                'record_name': self.record_name,

                'no_auto_thread': self.no_auto_thread,

                'mail_server_id': self.mail_server_id.id, 

            }

Via adding cc list value in this dictionary.
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