The Plus-up Payments are for the third EIPs - the $1400 payments - correct?   So your clients received the full amount allowed and are now receiving more?

This is correct.

are they SS recipients? IRS sent out double payments to lots of SS recipients when SSA forwarded them the direct deposit info and IRS failed to cross match who had already been paid...I wouldnt be surprised if they want that erroneous payment back

I believe you are referring to more 2021 advance payments for having children. Did these people file their 2020 tax returns? 

The $1,400 alone would have been for a single person. If children were added in the 2020 return, there is now an extra $1,400 for each child.

https://www.irs.gov/newsroom/irs-begins-delivering-third-round-of-economic-impact-payments-to-americ...

Cash the checks. because everyone knows that IRS really has a handle on this!