The error message, an Item with the same key has already been added, is caused by an employee or pay element having identical category splits.
If you don't know which employee or pay element is causing the error, raise a case referencing the title of this article. If you do know the cause, follow the relevant steps below.
Pay Element
Click Payroll, and then click Employee.
Find and select the required employee.
Click Standard Pay, then select the required pay element.
Click Open Pay Element, and then click the Categories tab.
Select the duplicate entry, and on your keyboard press Delete.
Click Save and Close.
Employee
Click Payroll, and then click Employee.
Find and select the required employee.
Click Open Employee, and then click Categories.
Select the duplicate entry, and on your keyboard press Delete.
Click Save and Close.