A.C.S. ACS legal Threatened to sue and made me pay over 500 dollars boston, Massachusetts

ACS called and told me that I was being sued for a cash advance that I never paid back. They gave me bank account info, ss#s, everything, so I looked at my account on the bank and I had paid it but two weeks later the money was put back in my account. So i thought to myself "yea, i guess i do owe them" so for the past couple of weeks I have been paying them money. Also, they would never send me any documents reguarding the matter. That should have been my first clue. Well, today, I made my last payment, 510.00, and one hour after I made it, a guy called telling me the exact same story again saying he was from ACS. We told him that was impossible because we just made our last payment and have been paying ACS and then he just hung up on us. Thats when i Knew it had been a scam.