When sending an e-mail, try not to use the "cc:" field unless the recipient in this field knows why they are receiving a copy of the message.
Using the "cc:" field can be confusing for the recipients. They might not know who is supposed to act on the message or what is their role with that e-mail.
Also, when responding to a "cc:" message, should you include the other recipient in the cc: field as well?
This will depend on the situation. In general, do not include the person in the "cc:" field unless you have a relevant reason for wanting this person to see your response.
If a copy must be sent to others, it is useful and gives a good impression, to explain why the e-mail is also being sent to others.
Remember that what is “obvious” to you as a sender, almost never is to your recipients, as mentioned in their frequent complaints about ineffective e-mails.
More recommendations for senders in my book Email at the workplace.