I filed taxes. I did not know that i had received $1400 stimulus payment/someone else got it. can't amend it until i know when i'll get taxes. what to do?

it asks when do i want to set up payment... how do i know when to put that date in for them to take it If i don't know when the deposit is going to be. why can't they just fix it... and send me the difference.   I never personally got that stimulus payment... therefore i didn't know until i had already filed taxes. then i got a letter. anyone else experience this?