But the December/January payments also count in the calculation.

they should have received the $600 each in January as well, if they didnt then they get to pick it up on the tax return.

You havent been asking clients about the second round of payments? What have you been entering on Line 19 of the RRC worksheet?

Perhaps it would help to review what is really happening:

The funds were paid out as Advanced payment against a projection. The projection used 2018 or 2019 tax returns. But 2020 is the Actuals. You use the 2020 return to reconcile what a person is entitled to, against what they got.

You might want to bookmark these links and read the IRS guidance.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-a-eip-eligibility

https://www.irs.gov/coronavirus/second-eip-faqs#Eligibility

One for each EIP.