Help with NCO zombie debt

About 7 years ago, I defaulted on an MBNA credit card. The account was taken over by Wolpoff and Abrams, who got an NAF award against me (grossly inflated with illegal attorney fees, interest, etc.). The award was stayed on condition that I make monthly payments. I paid them every month for a couple of years, then missed a few payments due to being unemployed. I then received a letter from W&A saying they were offering me an opportunity to settle the account for a reduced amount. I wrote back saying I was interested, but they never responded. In September, 2008, I received a letter from Daniels Law Offices, claiming they were collecting a debt on behalf of FIA card services. After several months of trying to get some sort of validation on the debt, I received a summons, to the effect that Daniels was suing me on behalf of FIA card services. After months of discovy, pre-trials, etc., they finally produced an agreement that I had with NAF, on behalf of W&A. I settled with the law firm, on condition that they would send me a written agreement, and I would pay by a certain date. If I didn't pay by the designated date, the case would go back to court and they would seek a default judgement. The agreement with Daniels took place in early November, 2009. Since then, I have not heard anything from them. Two weeks ago, I was contacted, via phone and mail, by NCO financial. They claim to be collecting a debt for FIA card services. It is obviously the same debt. I am assuming they bought the debt from Axiant, who owns W&A and is now in bankruptcy. I intend to send a validation letter to NCO, but I am definetely confused by the situation. How can two different companies try to collect the same debt? I also don't know if Daniels Law Offices is still in the picture. I am truly learning the meaning of "zombie debt," as this old account just won't die. Oh, just to pre-empt all the "just pay your bills and this wouldn't happen" people out there; first of all, no one likes to be unemployed. While I am working full time now, I wasn't when I missed payments. Second, it's difficult to know who to "just pay your bills" to when multiple people are trying to collect on the same old debt. I certainly don't want to pay someone, only to have some other bottom feeder saying I owe them money too. This is a confusing situation. Any advice would be greatly appreciated.