Smart Pension only allow each NI number to be used once. This can cause an error if you try to enrol the same worker twice (e.g. two contracts of employment), or if you are adding a new employee who already has a record on the Smart Pension scheme (e.g. someone who had left employment but has now rejoined the company).
You may need to remove an NI number from an old record in order to add a new record for an employee who has rejoined.
You may also get this error if the payroll ID (Smart's External ID) has changed and no longer matches between payroll and Smart. You should amend either in payroll or the External ID at Smart, so that these match.
If you need to change or remove any NI numbers which have already been supplied to Smart Pension this can be done quickly and easily on the Smart Pension website: