Do not cash the check.
The IRS has not issued guidelines as of yet. once they open on February 12 and see the situation, we are hoping for a resolution.
The Recovery Rebate Credit is the way people who were eligible for a stimulus payment, but did not receive one or received less than the amount they were due, claim their stimulus payment. However, if you have already received the full amount of your stimulus payment and also claim the Recovery Rebate Credit on your return, this will trigger a review at the IRS that could delay the time it takes to process your return and for you to receive your tax refund.
[Edited 2/9/2021|9:56AM EST]
Short answer, you may be able to pull your return back from Turbotax, correct that section of the program, and then re-transmit your return. Then it will be safe to cash the check, since you will have correctly reported that you received it, and aren't claiming it again.
If you can't pull your return back from Turbotax, then you will need to wait and see what the IRS does. They should correct your refund and reduce it by the amount of the second check, in which case you will know that you can cash the check. If the IRS does not reduce your refund, then you should hold onto the money since you should expect to be asked for it back.