Paid invoice handed over to debt collectors

Only 1 account was emailed in Dec and it went to junk mail. No follow up done by the provider. I found the email in September and called to confirm it was actually still outstanding, they confirmed it was. I settled on the day using payfast. On the day the funds reflect in their account they handed it over to debt collectors. Lancet confirm the payment has been received but debt collectors have not been updated and continue to threaten me. Can't get to speak to any manager at Lancet to get this resolved.