Actually it is not double dipping. Consider this example:
Client had Sales of 90,000 and expenses of 60,000. Received PPP OF 10,000 with forgiveness applied to 10,000 of expenses.
His book income would be 100,000 less 60,000 = 40,000.
Prior to CAA 21, his taxable income would exclude the 10,000 of PPP and reduce deductible expenses by 10,000. Therefore net taxable income would be 90,000 less 50,000 = 40,000. The net effect of the CARES was to include PPP in taxable income by reducing expenses. In that situation taxable income = book income.
Now under CAA 21, taxable income is 90,000 less full deductions of 60,000 = 30,000. This results in taxable income that is 10,000 less than book income. There is no double dipping; 10,000 received and excluded.
Hope this helps, this is how I explained it to a client.