New Ticket Creation or Append To Existing

New Ticket Creation or Append To Existing

If a customer sends in an email to our service desk what determines if a new ticket is opened of if that email is added to the conversation of an existing ticket. I had always thought the request ID had to be in the subject of the ticket. We just had a requester send in an email to open a new ticket. The subject of the email was the same as an existing ticket. In this example the requester was the same person. I want to be certain that an email from one account can not be appended to a ticket of another account. Please explain how this is determined.
Thanks

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