No, if you enter 2,400 and 1,200 for the 2 questions (or yes you received it) you should not have 3,600 on line 30. Line 30 should be blank.
Yes you can see the tax go up. Your 2020 return starts out by assuming you didn't get any Stimulus payments so it gives you credit for the full amount and your refund was too high. 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.
You are not paying tax on them and it is not deducting it from your refund or increasing the tax due. Turbo Tax started out by assuming you didn't get it and giving you credit for the whole amount so your refund was too high to begin with. Then when you entered what you already got the starting refund was reduced or the tax due increased to your real actual refund. Otherwise you would be getting the Stimulus amount again on your tax return.
Look at your return. The Stimulus payments do not show up anywhere as taxable income so you are not being taxed on it. And it's not being subtracted anywhere.
