Paid twice for one top up

I did an online top up for my LTE. after realizing that the the top up did not show, I then called in. I spoke to a consultant and sent the proof of payment. The agent was helpful and created a manual top up. However, at the end of the month they sent an invoice billing me for the manual top up. I then sent an email showing the previous correspondence and again attaching screenshots from my bank statement showing the payment being deducted from my account, in which I asked them to not debit my account with the already paid amount. I also advised that if the debit would go off, they would then either credit my account or refund the amount. To date I have received no response to the email or confirmation that they have received it. I also called in 2 days after sending the email and spoke to another consultant, who confirmed receipt of the email and advised that she would escalate it, but again, I have not heard anything from them or received any form of confirmation or acknowledgement of their error and the refund/credit