I didn’t receive my 2nd stimulus so claimed it on my fed taxes. Today, the 2nd stimulus arrived. What should I do with it? Cash it? Shred it?

The IRS records will show that your stimulus check was mailed, so you should cash it.  

You do not need to file an amended return now.  You must wait until the original return is accepted or rejected on February 12.  If your return is rejected, you can make the correction.    

Most likely, the IRS will detect that you received your stimulus payment and adjust your refund accordingly.  If the IRS does not auto-correct your refund, you should file an amended income tax return and pay back the difference.