I can see how frustrating if there's a duplicate transaction on your book. I'll explain to you the possible causes of why this is happening in QuickBooks Online Account.

An error might occur if the bank transaction isn't to your payroll or employees' paychecks. I suggest un match your bank transaction and re-match it. 

Here's how:

1. Go to Bookkeeping. Select Bank transactions or go to Banking.
2. Select the tile for the bank or credit card account you downloaded the transaction.
3. Select the Categorized tab.
4. Find the transaction you want to unmatch.
5. Select the blue transaction link in the Added or Matched column to see the matched transaction.
6. Select Undo from the Action column.

Once you undo it, This resets the transaction QuickBooks tried to create for you. Go back to the For Review tab and categorize the transaction.

Then, match the downloaded transaction with the one you already entered. It connects the transaction so that there aren't any duplicates.

1. Find a downloaded transaction with the Match option in the Action column.
2. Note the Date, Description, Payee, and amount spent or received.
3. Select the downloaded transaction to expand the view.
4. Review the Matching records found in QuickBooks. It is the possible matching transaction you already entered in QuickBooks.
5. Select the link next to each match to get more details.
6. Make sure this is the correct match. In particular, review the Deposit to, Payment method, and Bank account fields on forms for the existing transaction in QuickBooks.
7. If this is the correct match, close the open transaction.
8. Select Match.

I'm also adding this article for your guide on how to fix the issue on your reconciliation: Fix issues at the end of a reconciliation in QuickBooks Online.

Know that I am always here to support you. Please let m know if you have other concerns. Keep safe.