HANDED OVER FOR A SETTLED ACCOUNT !!!

I had accounts with the abovementioned company and I settled everything I owed them and I received a settlement letter from them. at some stage during the course of this year, they debited my account. I posted a complaint on this forum and a lady from I-smart with the name Kayla Peskin responded and apologized, stating that the debit was a technical error on their part. fast forward t today, the 5th of August, I received a call from this number *** and the guy told me he was calling about the amount of R10 000.00 that I owe to iSmart !!!! Like really ?? when I called the number I realized it belonged to a firm of Attorneys based in Nelspruit. I would like to know how this came about as I settled my bill with I-smart a long time ago. someone please explain this to me.