Pathetic Service

I purchased a shirt on my account the 17 June at Truworths Canal Walk for gift and in the 1st July I returned it for a bigger size also any the Canal Walk branch. Instead of crediting my account of the shirt to void it the cashier scanned it again so the shirt was charged to my account again. Me thinking the shirt has been voided from my account gets the bigger size and goes ahead to have it added to my account but only for the cashier to tell me that I have insufficient credit on my account. My mother then decides to take the shirt on her account. When I go home that night I only realised that the shirt was scanned in my account twice and once in my mothers . So I actually paid thrice for one shirt that I purchase. On the 2nd July I went back to the store to show them the proof and they managed to credit me once as the second purchase hasn't reflected yet. Now on the 27 August that one shirt is still reflecting in my account and I refuse to pay for it because it reflects k my mothers account already. I've been countless times back at the store and they keep telling me that a complaint has been made to head office but no has gotten back to me since. It's almost been 3 months and their credit office keeps calling me to say that my account is in arrears and I just keep telling them the same story. I am so sick of this issue and I'm afraid that it may lead to me being blacklisted due to people not doing their job properly.