With the Scheme rolled out at pace, plus change to the rules along the way, errors can creep into claims.
HMRC may charge penalties if you are overpaid and fail to notify it of the fact within the correct timescale. Notification should be made within 90 days after the date you receive an amount to which you were not entitled. HMRC states that it is not looking for innocent errors or small mistakes. Where someone didn't know they didn't qualify for a grant when they received it, HMRC will only charge a penalty if the grant is not repaid by 31 January 2022.
There are some key areas worth revisiting. Check, for example, that there isn't a significant discrepancy between the amount(s) HMRC has advised you were due, and amount(s) received. Overpayment could potentially occur if you have a business that has permanently stopped trading; or (because companies aren't eligible for SEISS) if you have incorporated a sole trader or partnership business since 5 April 2018. Or indeed, if you misunderstood the rules and claimed when you weren't eligible.
The process to notify HMRC of an error, or to make repayment, involves completing an online form, then making payment. HMRC is unlikely to contact you unless it needs more details or there is a problem with your payment. The detail is here.