Double interest

I bought a phone on account in October 2015 for R3000. I started paying the phone immediately and I didn't miss any months. To my surprise, I was surprised to receive a statement that I have been charged interest which is no issue. My issue is how can I be paying for a phone and be charged close to R3000 interest again? If I only knew that Edgars cheats people out of their money like this, I wouldn't't have bothered!!! So much for a reputable company!!! You guys are real disappointments to the retail industry.