I know a way on how to manage duplicate transactions in QuickBooks Online, VM15.

The best way in handling duplicate transactions in QuickBooks Online is by excluding them in the program.

Before we exclude them, we should compare the two transactions to ensure that they're duplicates. Following verification, follow the steps below to complete the excluding process. 

1. In the left navigation tab, click on Transactions and choose Banking.
2. Locate the credit card account.
3. From the For Review tab, select the duplicate transactions.
4. Hit on the Exclude button.
5. If you need to delete them in the program, just go to the Excluded tab, select the duplicate transactions.
6. Hit the Delete button.

To learn more about this process, read this article: Exclude expenses from downloaded bank transactions.

Additionally, you might be interested in the following articles below. These will give us advice and step-by-step instructions on how to manage bank transactions, as well as reconciling an account in QuickBooks:

Tips for Working with QuickBooks Online's Bank Feeds: Matching Transactions.

How to Reconcile in QuickBooks Online.

Let me know if there's anything else you need help with reconciling credit cards in QuickBooks Online. I'll be right here to assist you anytime. Have a nice day, VM15.