A PNR is not supposed to be unique. It literally is a record, that is tied to the airline ticket stock code (first three digits of a ticket number) and booking system (Sabre/amadeus) until recently a gds was not able to query one or the other (and yeah there are more, even ticketless/couponless ones)

Thats why you are required to have two to verify, ticket number or last name but in old old systems you always used the ticket number as that had all the passenger information, coupon status, route, etc the PNR is just a shortcut to facilitate this.