northland is double charging on paid accounts

Northland was collecting a debt that I had with capitol one on two credit cards.I paid them off in 2004 and they sent me a letter of satisfaction.In April of 2008 they sent me a bill for the exact same debt.I faxed them a copy of the satisfaction letter,and I thought that would be the end of it.Then they sent me another collection letter September of 2008.I called them and they replied that I only paid off only one of the credit cards.Both cards totalled $700.00,the collection was $1750.00,which I paid.Now they are trying to charge me again.Can they really get away with this.What can I do about it?