https://www.irs.gov/coronavirus/second-eip-faqs#reconciling

in the information under  Payment Issued but Lost, Stolen, Destroyed or Not Received

you can enter $0 for EIP#2  -  IRS will then initiate a payment trace.   In the meantime, taxpayer will receive a letter the RRC was changed until the payment trace is completed.  Once complete (and the EIP#2 was not cashed)  an adjustment will be made.

I gave this information to my clients when they picked up their returns so they would know what to expect.  A long drawn out wait but still hope to see the money.