Paid up loan - now called to say im still owing - and i have the paid up letter

I write this review, and highly upset. I had 2 RCS accounts. one was a card and the other was a loan. I was handed over on both of these due to financial difficulties. Which in this day and age is common. I then 1st paid off the RCS card, I then paid off the loan as well through a well know collection company and received my paid up letter from them as well. Now 2 months later I received a call from another Collection company advised that I'm still outstanding on my loan. How does this work. How *********** is the people at RCS or the the collection companies. This as if stress isn't enough already