You need to enter it to see if you qualify for any more and so Turbo Tax doesn't give it to you again. The Stimulus Checks are really based on your 2020 return we are filing in 2021. So any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.
Your 2020 return starts out by assuming you didn't get any Stimulus payments so it gives you credit for the full amount. Then near the end you enter how much you actually got so it only gives you the difference if any. So you don't get it again
