Double debit

My debit order for March did not go through. I paid it then with a transfer to cell c. End of April they took a double debit and when I phoned they said they did not see that I'm in credit and that's why they took a double debit. They said they can not pay it back but will keep it in credit and then I don't have to pay this Month. They now again took a full debit, this never happens at our other service providers. Please pay my money back into my account as my other debit orders needs to go through!!