Over charged

In December I was billed for my contract phone as usual and money was taken out of my account as usual then early January I got a call saying I defaulted my account when I inquired their agents told me I don't owe Vodacom as they made a mistake by double billing me but the calls and sms didn't stop to say I owe them then this month today infact I've been double billed and I feel it's unfair for me to run around sending them proof of payment when I've never defaulted them in any way before so I would appreciate it if they fixed their mistakes without me having to skip work and spend my petrol to fix their error and I'd appreciate it if the also pay my money back which is an inconvenience to me right now and also pay my bank charges because it's not my error it's theirs