ER Consulting/NuDebt Forcing Me To Pay A Settled Bill

I've received an invoice via email in December last year from ER Consulting. I was rendered confused as I recall settling the invoice upon my arrival at Casualties. After I stated that the bill was settled in cash on the same day, they requested proof of payment. I advised them that I no longer have those receipts, as to my knowledge, the bill was settled 10 month's ago at that time. I also questioned, why is that I'm only receiving this email now, towards the end the year? Their response was that they had a 'system glitch' and communication was only sent in October last year. I had no response as I was bewildered by this whole situation. Last week, my sister sent me a screenshot of an SMS that ER Consulting have sent to her, stating the outstanding balance must he paid by a specific date, which was the next day, failure to pay will result in my account going to NuDebt and damage my credit bureau report. I immediately sent an email to ER Consulting stating that I have previously advised that the bill was settled ON THE DAY of arrival, I do NOT have proof of payment as it was a CASH payment and I do NOT have the receipts anymore as to my knowledge everything was SETTLED prior to these recent emails. They responded, 'Kindly be advised that we require proof of payment'... I was LIVID and reacted immediately, I requested to speak to their superior which to this day I have not received a response. I REFUSE to be held responsible because of ER Consultings 'system glitch' and be forced to pay a SETTLED bill just because I don't have the receipts which was way over 6 month's old at the time when discarded. ER Consulting, this is UNACCEPTABLE! Now I see this morning, NuDebt has affected my credit bureau report because of this. Please fix this!