Letter 6475 is just telling you what the IRS sent you for the 3rd Stimulus back in March. It was 1,400 per person. Check your bank statements if you got it. Then you need to reconcile it on your tax return on line 30. Do you have an amount on line 30 for the Recovery Rebate Credit? Line 30 is only if you didn't get the full amount or qualify for more. Sounds like you may have tried to claim it again on line 30.
If you didn't get the separate stimulus payments that the IRS sent you then you need to put a trace on it.
Request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
