Billed again after being billed mid FEB

I am really annoyed by netstar. I missed end of Jan payment and come mid feb my account was debited double. I called to find out why the double debit and was told that it is because they also debited the End of Feb amount and the agent said that he doesn't understand why this was done hence he assumes the double is because of the end of feb installment. I left it at that but lo and behold I just got another debit for end of FEB. This is not the first time i get debited an incorrect amount that sometimes leave my account in a bad state. The last time i was promise to be refunded which I only got half of my money. Is there someone who can thoroughly explain to me why I have to be billed twice in a month? Is there a way I can pay this account myself? I seriously cannot wait for my contract to expire so i can move on from Netstar. I am tired of having to pay for their system errors or whatever causes them to make such mistakes without having any explaination or reason.