Product: Credit card or prepaid card

I am having problem getting my money back from the Saks and its credit card management company Capital One while the company incorrectly posted credit to my regular Saks store account and therefore overcharged me. It has been almost two months since I first called the Capital One and it has been trying to cover up their mistakes without giving me money back. 

I am writing the complaint to ask for my full refund as well as interests owed during this period for overcharging me. 
( the " company '' below means Capital One ) In XX/XX/XXXX, I called the company and asked for a refund check for credit balance on my account of {$830.00}. I have received the check and have cashed the check only once in XX/XX/XXXX. However, the same credit was posted to my account twice and therefore my account had a debit balance that I had to pay. I have been calling the company at least 3 times regarding the issue yet the customer service kept changing the story and today is XX/XX/XXXX and I still haven't seen my overcharged balance being refunded. 

Below are the summary of events for how my Saks store account was overcharged which can be seen from my statements : - The request was processed and posted to my account with transaction date XX/XX/XXXX and posting date XX/XX/XXXX. This showed up in my XXXX statement. 

-Somehow another credit of {$830.00} credit refund was posted again on XX/XX/XXXX with transaction date of XX/XX/XXXX. 

-Separately, I returned an item on XX/XX/XXXX for {$570.00} to my Saks store account and posted on XX/XX/XXXX to my account. Due to the duplicated credit posted to my account from the company 's error of processing, my account had a debit balance of {$260.00} as of end of XXXX and I had to pay {$260.00} rather than a refund of {$570.00}. 

The responses from the company are as follows : - XXXX : I called the company on XX/XX/XXXX and asked for duplicated credit of {$830.00} posted to my account. Customer service insisted two checked have been sent for the same amount and therefore my account was debited twice. I insisted that I received only one check and I cashed only one.  Customer service later said she would file an inquiry so the back office could research and get back to me within a few days. 

-XX/XX/XXXX : followed up with the company as no one called me back. Customer service said case still pending. 

XX/XX/XXXX : followed up with the company as no one called me back. Customer service said case still pending. 

-XX/XX/XXXX : My statement of XXXX came and still no sign of reversal of erroneous credit posting to my account. 

- XX/XX/XXXX : I called the company again and asked to speak with a supervisor. The rep again said investigation was still pending after more than a month of investigation! Yet she said notes from my account said two checks were issued but the second one was never mailed out when somehow the second check was deemed duplicated. She said she would follow up with back office to see why my account still hasn't been refunded since the duplicated credit was never mailed out. 

-XX/XX/XXXX : The customer service supervisor called me back and gave me updates : While my account was debited second time in XXXX, the check was never sent out. After I kept calling the company, there was an expedited request put in by back office ON XXXX, XXXX, XXXX to mail the check out. She said I will likely receive the check before end of XXXX. 

So it turns out the company has never done anything with " investigation '' and never bothers to correct any mistakes. In addition, even with this {$830.00} check, the company still owe me {$260.00} that they have overcharged me.