Wonga is glitching and not responding.

I took out a small loan from them beginning of December. There was a glitch on their system which resulted in a double debit the end of December. I got a refund back for the second debit a week later. However, they did not fix the glitch and it shows there is an outstanding balance and have handed me over for collection, even though I am up to date with my payments. There is no response from them after many emails sent about the glitch, but I did get one response which said I had an outstanding balance, so my email was not even read by whoever responded. Every day I speak to a different person from the collection agency and explain the story, they keep calling. Wonga is not responding and I am worried about my credit and if another double debit will be taken off this month, which affects everything.