Formal Billing Dispute: Duplicate Installation Fee

I am writing to formally dispute an incorrect charge on my recent bill.

1. The Issue: I am being charged twice for a single Professional Installation. 

2. The Facts: I only had one installation appointment, caused I made the one on Jan 28th then reschedule this appointment to 30th, However, my bill showed two separate installation charges. One charge was previously waived, but an extra duplicate charge still remains on my account.

3. Previous Communication: I spoke with a representative who claimed they "cannot waive it a second time." This is unacceptable as this is not a request for a discount, but a correction of a system error and duplicate billing.