Standard Bank Charge of interest twice

Any one that can offer guidance on this matter:

A few years ago i took out a student loan with Standard Bank . We were then advised that while i am studying my parents would pay of the interest charged on the capital borrowed and then as soon as i start working i will then pay of the balance of the capital remaining after interest was paid . We paid over 3 years a total of R18 000.00 worth of interest .

Nether the less i started paying of the the balance of the capital for the last 2.5 - 3 years. Every month i am still being charged more interest on the capital.

Which means i have been paying interest twice on the amount borrowed . The student loan amount was for R50 000.00 in total up to date INCLUDING THE INTEREST MY PARENTS PAID AS WELL AS THE EXTRA INTEREST I AM BEING CHARGED i have paid R26 000.00 worth of interest which means i still continue to pay interest until the balance is settled which will only be mid next year .

I would like to now how on this **** earth can you pay so much interest on a student loan of only R50 000.00 which means i have almost paid double the amount i have borrowed .

STANDARD BANK IS MONEY SHAR****!!! I have called the call center numerous times trying to track down the contract i have signed . I have gone to the branch where this contract was originally signed without any luck in why i am paying interest twice on one amount borrowed.