Paid bill in 2018 yet still being harassed and receiving invoices

On 21 Nov 2017 I had a biopsy at Groote Schuur Hospital. I was totally unaware of any outstanding bill (as did not receive any letter, email, call nor sms) until 25 April 2018 when i was called by MBD call centre (debt collectors!!!) My account had been turned over to them without even been notified of the account in the first place. I paid the full amount owed on 30 April 2018. I corresponded per email with Jeffrey Ngoetjana at the finance department of NHLS who assured they received my payment and cased closed. Today I receive a posted invoice dated 24 December 2019!!!!! for this same biopsy of 2017!!!!!! I am someone who always pays my bills so I find this totally digusting!!!!