"Client received multiple1099-misc for payments and some were made by credit card, so there is duplicate income reported."

Neither the 1099-Misc or 1099-K is the business income to report for taxes.

1099-Misc is only issued by those clients who paid your client $600 or more. Your client can have had $2m in income from so many clients, that none of them had to pay $600 or more, so there would be No 1099-Misc issued by anyone. You still report the $2 million, of course.

1099-K is a Payment Settlement reporting entity that has handled Money; this is more like Banking, not sales or income. For instance, a Landlord might have a lot of 1099-K activity for Rental Deposits, which is not income.

Income is the business activity as a whole.