Error creating pay history record: Violation of UNIQUE KEY constraint 'IX_Pay History'

If you encounter this error message when trying to create a new pay history record for an employee, it means that the record you are creating is not unique to that appointment.
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This is denoted by two fields in particular:

These two values must be unique to the appointment, so, for example, you can't have an employee with the following record more than once in a single appointment:

You can either update the existing record if it is incorrect, or create a new pay history record with unique values (e.g., a different pay from date or pay element type).