National Credit Systems, Inc. Shady, Rude, Secretive and Dishonest atlanta Georgia

got al letter from these people  about a debt supposedly owed to an apartment complex i formely lived at. thing is the debt had already been through collections with judgements and garnishments. and finally a stipulation until paid in full. which it was, paid in full for almost a year. anf i get thsi letter from national credit systems. they tell me nothing when i call wont help wont say where it was filed from nor who filed it. i , on the other hand have all the documentation  of court records and pament reciepts  showing a paid in full  release..why is these peole trying to scam me and get me to pay for a debt i already paid for is beyond me.  i am goning to sue them, i think they r fraud and scam