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.
