Requests Created By Email

I am building a queue to help triage our countless emails from our email defender system.  

When the system sends the message The sender and subject are always the same. 

I have an automation rule set up that on create the service request is set an then there are additional labels based on the body of the email.   Many of these emails can be largely identical in content since they are system generate. 

This is working most of the time however sometimes a request comes in and it updates an existing item instead of creating a new one. 

The question is what is being tested to determine if it is an update or create

Hope that all makes sense

thanks in advance. 

greg