Spirit Airlines Double-Charged Baggage Fees, Refused Refund

Last week, during my round-trip flight from Texas to Florida on Spirit Airlines, I was charged baggage fees twice. I had already paid the luggage fee on the website after booking the ticket, but the agent insisted I pay it again before boarding the plane. I explained the matter and attempted to express that I had paid using my credit card, but they refused to listen. They said their system showed no payment and told me I couldn't board the plane unless I paid again. I reluctantly paid for the second time. The next day, when I called customer service to request a refund, they informed me that the investigation was still ongoing. The customer service and ground crew were very terrible and extremely disrespectful.