When running the Validate Payroll Data report, if there is a duplicate employee with the same Employment ID in both records you will see the error:

'Employee (XX) in Payroll XYZ has an Employment ID value of (YY). The same employee exists in this payroll (Employee XX) and has the same Employment ID.'

Or

'Employee (XX) in Payroll XYZ has an Employment ID value of (YY). The same employee exists in Payroll "Monthly" (Employee XX) and has the same Employment ID.'

If an employee has multiple employments/roles in the company, they must have a different Employment ID for each one.

To resolve this error, you must edit the Employment ID in the Employee record.