First, if they cannot be claimed as dependents by you then you simply don't claim them on your tax return. They will then file their tax returns to establish eligibility for the Recovery Rebate Credit based on their tax year 2020 returns. Here is a link with more information on the Recovery Rebate Credit.
Next, There is currently no specific guidance on offsets for the Recovery Rebate Credit. However, under the Coronavirus Response and Relief Supplemental Appropriations Act signed into law on December 27, 2020 the second round of economic impact payments (stimulus) will not be offset for any reason.
[Edited 1/25/2021, 2:45 EST]
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