What should you do if both parents who are not married but take turns filing the child every other year both got credit ($500) for the child?
The Internal Revenue Service (IRS) says to do nothing (FAQ #30). There is no clawback provision in the CARES Act, so math error in your favor.
That said, each parent should receive a letter from the IRS (Notice 1444, Your Economic Impact Payment) within 15 days after the payment is made. The IRS advises you to keep the notices for your 2020 tax records.