Nu Debt takes on Old Debt

Nu Debt is demanding I pay a Telkom bill from 2019. Telkom discontented my landline service in 2019 and failed to provide me with an alternative wireless service despite numerous documented requests by me, and continued charging me monthly, part of which I paid trusting them to deliver a wireless phone. The "debt" of R1030.40 was handed over to legal firm Blake and Associates and after much paperwork and proof from me, the case was withdrawn and closed by Blake. However, I am now receiving threatening emails and calls from Nu Debt management for exactly the same debt. I do not want to go through the mammoth task of proof again, but Nu Debt don't care. Lastly, is this email legitimate? ***