WebADI, how to create attribute validation that verifies Excel cell entry of national_identifier doe

I have created an integrator with an interface that uses the procedure HR_Employee_API.Create_Employee or Create_US_employee, however the procedure allows for duplicate entries of national_identifier/SSN. The People form allows duplicates but gives a warning only. I am trying to create the interface attribute validation to check for existing national_identifier numbers against the entry in the cell in the upload spreadsheet. With validation, I want the upload to error on the row for an existing NI/SSN (or Enter a valid SSN).  I have been using the Attribute Validation Type Table and the validation entity per_all_people_f papf, however I have been unsuccessful at