POP was send but getting threats after 2 years

After 2 years, my husband received another email to pay an outstanding amount of R200 that was since paid. Medicross Meldene had a debt collector called Vericred, which we paid,but according to Sibongile at Medicross they never paid the moni to them. Who's fault is that. Now they have a new debt collector an sending us this bill. They need to collect this money or open a case against they provider that never paid them. I send proof to both the old an the new debt collector but still I'm being send the same bill. After 2 years can you really ask us again for what was paid. Its not my responsibility to act on they behalf to find this money, they were in partnership with Vericred an its their responsibility to follow up with them. Acc MEL***