Flora,
Unless I’m missing something, you may be replying to the wrong idea. 
The problem here is that people CC’d on emails are automatically created as new contacts.
The solution you mentioned appears to be a plug-in for Chrome that allows logging and tracking emails to a contact record (sort of the opposite of what we need).
It has been a few years so it would be nice to hear something from HubSpot on this.
More specifically, if:
1. we send an email to a client, then
2. they reply back and CC their colleagues
… we have no way to reply all to the thread and log the email to HubSpot without creating the colleagues as new contacts.
One idea is to not auto-create contacts who have been CC’d, or at least make that an option.
Thank you for your interest here. Could you knock that out real quick for us?