I was looking in Setup>Tickets>Options> in the General Options section and notices the setting:
"Allow Non-User CC Updates" = Indicates whether to allow Ticket updates, via email, from people who are not helpdesk users, but were CC or BCC recipients on the ticket update email.
Also under the Client Option Section,
"Clients Can CC; Email" = Allows Clients to carbon copy email Tickets to additional addresses.
If the Cc is causing issues then you may want to consider restricting who can Cc and when using these settings. From what I understood, the incoming email account is supposed to be limited to submitting tickets. Once a ticket is created, then from inside the ticket is when and where a CC or BCC should happen, not during the creation of a ticket. this would be like creating a CC to a printer while sending in an email ticket which would have the potential to create an infinite loop given the right setting configurations and I have seen the system generate thousands of tickets as a result of this practice under certain conditions. I would just hold the CC and BC activity until after the ticket was created and do it from inside the ticket.
Just my 2 cents, hope it helps or gives you some ideas.