Here are some thoughts:

1/ Hopefully they wrote the correct SSN(s) on their checks, and hopefully the IRS took note of the glitch and attributed their payments to their account.

2/ I would call the Practitioner Priority line and check to see whether their account reflects their payments.

3/ The clients should be cautioned to keep their bank statements to document the payments if that should become necessary.

4/ The clients should be encouraged to set up an account at irs.gov, where they can view their account and payment history.

5/ The clients should be encouraged to make their estimated tax payments online at irs.gov to avoid these types or errors and mishandling of payments.