So check your 1040 line 30 and see it is lists 1,400 for the Recovery Rebate Credit . The IRS says they already sent it to you but you tried to claim it again. Did you get IRS letter 6475 saying how much they sent you?
Check your bank statements or Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get 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...
