Once an account is sent to collections and settled, can the company that sent it there legally continue to bill you?

I had a personal training membership with fitness connection. I wanted to cancel due to financial hardship. I told them if they would cancel I would pay off what I owed, since I breached the contract. They refused and I was unable to pay at the time. When I was finally able to make payments, I took my overdue bill to the gym and set up a payment plan with the manager and he finally agreed to cancel my account with no further charges. I made my first payment and one week later they sent my account to collections. I called and they said to deal with collections so I said okay. I have set up a payment plan with first credit services and they have been very helpful in working with me. However, I have recently learned that my account at fitness connection wasn't ever canceled and I am concerned that even if I settle with collections, that fitness connection will still continue to bill my account and then send that to collections. I have a written statement from the general manager stating that my account was cancelled and no charges would be added. It is not notarized and I am concerned that I could wind up paying more.