Duplicated Earnings - PAYE

I have received a Bill for Tax upon investigation it shows my previous employer duplicated my earnings, I contacted HMRC with my p45 and payslips and i received a letter back agreeing it's obviously a duplication, however my previous employer has to rectify the matter, the long and the short of it is, he's either unable as he's not very tech savvy or although I doubt it, not acting out of malice. I received the original request back in about August 2022 and left it with my employer saying to leave it with him, June 2023 i get another demand, I write to hmrc again recorded delivery contact my employer again and get no response from either. Foolishly thinking that after a few weeks of nothing it's all sorted, it goes to the back of my mind. get another letter this week now threatening debt collectors, my employer has stated his software company has no idea how it's happened and can't rectify or duplicate the issue and he would like to send written records but they wont be accepted. I've tried to get through on the phone but it's impossible. I don't owe this tax, I feel like I'm being held to ransom due to my employers Admin error and subsequent lack of action to rectify it despite being able to prove my earnings. where do i stand trying to fix this, am i going to have to find the money to pay it and claim it back hopefully later or what?

Hi Maria Bowen,
We are sorry to hear that you're having issues with this duplication of employment.
Please ask this employer to urgently contact our Employer's Helpline on 0300 200 3200
We should be able to advise them on how to resolve this issue.
Thank you.