Debt Collection

Nbd Debt Solutions called us and stated that we were handed over for an account which was settled in February 2018. We tried to explain that this was settled but they refused to understand saying their client is right and we are wrong. We went into the "clients" offices requested a full and final settlement figure got it and paid it in cash, now 6months later we are being contacted and said we still owe some monies?? Gave us an email address which doesn't even excist because emails keep coming back? Why do some debt recovery agents always think that the consumer is the one that is wrong??