Can I merge same Person, same name but with diff. Person number?

Summary:

-Personx is an internal applicant who does not have an email address and therefore he set up a candidate profile and applied for a new role using his personal email address as if he was an external applicant.

-Personx was successful in the recruitment and we put him through the offer letter phase etc and then have moved him to HR.

-We assumed a duplicate check would run at some stage (believing that this compares first name, last name and DOB with existing or former employees) but he changed from a pending worker to an employee and no duplicate report ran.