Thanks for the response @SashaMC . As you said that solution works if the transactions haven't been added to the register. Unfortunately the client had categorized all the transactions, including the duplicates. I went through the process of deleting each of the 500+ duplicates one by one. 

The client has multiple credit cards and experienced a staffing turnover, so I can guess how this could have happened. But I want to believe that a digital program like QBO should have a better option to clean up an error of this size, especially for the accountant platform. I could have zeroed out the duplicate credit card balance by journal entry, but the vendor activity would reflect inaccurately if I did.