So, let me understand this…
‘customer’ sends email like this…
CC: firstname.lastname@example.org, email@example.com
The email server at firstname.lastname@example.org receives the email. RT via fetchmail (or whatever) then gets the incoming email for email@example.com from the mail server.
RT then sends an email to the queue watchers (typically as a ‘comment’ transaction which includes a link to the RT ticket, and ticket ID in the email subject right?); user1 and user2 are members of the queue watchers.
User1 and/or User2 starts up their email client - download emails.
They receive an email that the customer sent them dirrectly as a CC (which bypasses RT).
They receive an email that RT sent to them as a ‘comment’ transaction.
Can you now see how RT can’t be responsible for, or fix this?
You could write a scrip to prevent RT sending ‘comment’ emails to queue watchers CC’d, however then they will not know the ticket number - which is useless in my opinion because your watchers are no not using RT, but bypassing it.
The problem is at the mail server and mail client level, and I suggest you fix it there. E.g. setup some email rules so the users are not sent emails when CCd and To is firstname.lastname@example.org.