Hello, it's a problem that concerns only the "Sale" module, for example what you said does not occur in "Purchase".

The emails of estimates, orders and invoices provide the sending of the default mail to the customer, you can enter other recipients but the customer remains in the same copy; the other recipients and the client become 'Follower', (you can see it in the drop down below the order). You can delete followers if you do not need to copy them anymore, remember that since you keep them all the emails will arrive.