Person Number generation difference between rehires and existing employees

Summary: When ex- employees are applying to a job via external career site, system is unable to identify them as ex-employees and when existing employees are applying via external career sites , system isn't able to identify them as existing employees

Content (required): So, here there are two scenarios,

When i am applying as an ex-employee, like taking the first and last name of terminated employee , and applying via external career site using a different mail id, system isn't able to identify , this candidate as an ex-employee. it just says external candidate. So, when i am hiring this candidate a different person number for this employee is generated, not the last one . This is an issue , right?