NTO uses data extensions to manage the subscriber information usedfor their email sends, and those...

NTO uses data extensions to manage the subscriber information usedfor their email sends, and those sends includes calls to update records with new or different subscriber information. The developer handling these records writes some AMPscript to check and see if the data extension containing those records updated usingan InsertDE() call if the record doesn't yet exist. Why would the developer receive an error stating the application cannot insert a duplicate value for the primary key in the data extension?