Dealing with a Collection Company named ERC

Back in 2002 I had a speigel account with a balance of $320.00. Spiegel had informed me that they no longer were using the credit company that my Speigel Card had been issued through so I paid off my account. So I thought. I became very ill shortly after sending what I thought was my final payment on my credit card account. Never received anything from Speigel and I received a phone call about a year and half ago from this company stating that I owed Spiegel $1,162.00 which I told them I did not owe this amount and to stop calling. They did and today I received call from ERC advising that Speigel wanted their money. I explained to the collector that I did owe Spiegel as I had paid off my account. They want proof of this payment. I changed banks several years ago and I do not have that information and the bank told me it would be impossible for them to find that particular check as I wrote over three years ago last month. ERC advised that Spiegal had charged off this debt in 2003/2004. What do I do. I live in Arizona and I know that the collection statute is three years. I have no problem paying my original balance owed at the time I sent in my final payment. THe amount they are trying to collect is over three times the original debt. Any assistance? I check my credit report and I only located the charge off reported on one of the three credit agencies.