Hello,
As a starting point, I agree with Fortune, you should only be liable for the net amount, that you received.
I would seperately contact Revenue and find out how you go about having any tax credits that there applied to these two "salary" payments, reversed.
What have you done with the two payments ? Whatever about the first one, you knew you weren't supposed to have received the second one, so I assume you still have it, and could return that, quite quickly, yes ?
You could then offer to return the first month's payment over an appropriate period, based on your ability to repay (while I wouldn't put myself under particular pressure with these repayments, I also wouldn't "extract the Michael").
You've done the right thing by contacting them, and acknowledging that its not your money to keep. Sadly, they've responded badly, but don't let that stop you from being an honest and decent person.
Also, is there any chance that you may need a reference from this party, in the future?