State tax Question?

About 6 months ago my wife and I got letters saying that on our 2004 taxes we got back more on our refund than we should have. It said that we both got back over $400 a piece. We have never gotten more than $200 a piece before we got married and since we did our taxes this year "married filing jointly", we got just under $300. How do we prove that we never received the amount they said we did?