I'm getting another refund after amending. Do I need to send the first one back?
No. You just have to wait until you receive the first refund before you file the amendment.
Once you have your refund, the money is yours. Don't send it back, and you don't need to wait for your amendment to go through before using it.
After the government processes your amendment, they'll send you a check for the second refund. This can take 3-4 months.
Is the second refund in addition to the first, or does it replace the original refund?
The refund on the amendment is in addition to your original refund. So if you originally got a $2,000 refund, and another $500 after amending, your total refund would be $2,500. More info