Actually, it's not to prevent you from getting it twice. It's to ensure that if you didn't receive the full amount that you are entitled to based on your 2020 income that you will get the missing money added to this year's refund.
This can happen if you didn't get your payment at all for some reason or if you circumstances changed and you would be entitled to more money now than was advanced to you previously - less income, different filing status, a new child, etc.
