Smart Pension only allow each email address to be used once. This can cause an error if more than one worker has the same email address in payroll, 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 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 do have duplicate email address the only way to correct this is to provide unique email addresses for each employee. If you need to change any email addresses which have already been supplied to Smart Pension this can be done quickly and easily on the Smart Pension website: