A user has send an email to our helpdesk.
This email is converted into ticket [INC-20557].
The user recieves an email telling him about this ticket.
The user forwards this email to the email address of one of our agents.
The agent sends a reply to this user with our helpdesk in CC. The subject at that time is 'RE: [INC-20557] FW: laptop/phone/kaartjes'.
This reply has created a new ticket with the agent being the requester. The agent received an email with the subject '[INC-20617] RE: [INC-20557] FW: laptop/phone/kaartjes'.
Sounds like a bug to me...