Unexpected paper refund check

I have received an unexpected refund check from the IRS, I wrote VOID on it sent it back along with an explanation.  Then I got a letter from them saying that they are processing my letter. Then 2 months later a got the same refund check again, only with more interest paid.  What should I do?  I don't really think i should be getting a refund. The amount is sizable ($6000). I think I am stuck in some kind of loop with them. If I call them, I get a customer service person that does see anything wrong. (In other words, useless in solving my problem)