Bank account debited twice on instalment payment - Not reflecting on store account statement

The cashier at the Daveyton Mall debited my account twice (R 300.00 x 2) upon paying my instalment at till-point on the 5th of May 2020. I consulted the store to get refunded one R 300.00 and i was placed on a telephone call with their customer service agent. The agent asked me for proof of this transaction in the form of a bank statement and my ID copy for which i emailed to the email address provided on call. Today I receive a store account statement reflecting that only one payment of R 300.00 instalment was received by the store. The other (additional R300.00) does not reflect. I am being charged interest on a high amount which did not take into account the extra amount paid. This is just exhausting and wish i could get feedback from them in all my communication.