If you have already filed and your return was accepted, wait for the IRS to process your return and then file an amendment and claim the Recovery Rebate on Form 1040X.  If you have not filed yet, you can add the Recovery Rebate for the second payment before you file.  It will show up on Line 30 of your tax return Form 1040.   To claim the Recovery Rebate in TurboTax - -

Go to the search bar up top and type in stimulus (make sure you are all the way inside of your return or search does not function), hit enter and then use the Jump to stimulus link to be taken to the questions.   

I think you are saying you said you didn't get the 2nd Stimulus payment but you did get it?  So you have 600 on line 30?  Don't do anything.  The IRS knows they sent it to you and will adjust your return.  It will take longer to get the rest of your refund.  Or if if line 30 was more than your refund they will send you a bill for tax due.