Debit paid but still reflecting as a bad payer by Capitec Bank

Last time I used their overdraft facility and paid on time but they said I’m suppose to pay when I paid already. It was December and we got payed on the 15th before I knew it they already deduct money for the overdraft that we agreed on. Comes the 1st of January they tell me I owe them for December and that I must make arrangements with them to pay the instalment for December when they the ones who debited on the 15th of December when I got paid. The question is how do I owe them when they already paid they self by deducting on the 15th of December and their debit order date is the 1st of each month and worse the was money on the account why didn’t they take the money and I pay double for that month because that’s what they wanted from me. Last I checked at Transunion I’m being label as a bad payer because it’s show I missed a payment for that month of December. It’s affecting my credit because they say I must spark to them to fix it. How do I go on and fix it? This big companies have a tendency no to accept when they did you wrong and can’t fix that so is there a way they can fix this and if not how can I force to to fix it. I’m disappointed at my bank to think I’ve paid them then to find out that I’m being treated as a bad payer.