Hello there, wombatlawnandgar.

Let's make sure you'll be able to delete the duplicate expense in QuickBooks Online.

If you mean deleting the transaction that you've manually entered in your book, we can simply follow these steps:

1. Click the Search icon at the upper-right corner.
2. Hit Advanced search at the bottom of the pop-up window.
3. Select Expense as a type of transaction. It'll let the system generate all the expense transactions that you've entered in QuickBooks.
4. Click the Search button.
5. Hit the Date Filter drop-down arrow. Select the appropriate dates.
6. Then, Apply.

Once done, look for the duplicate expense and open it by clicking. Within the Expense page, click More at the bottom part. Then, Delete. That's how we can remove duplicate transactions in your book. When they are deleted, they will not affect to your reporting side.

On the other hand, if you are referring to remove or delete duplicate transactions from bank feed, we'll have to exclude that expense so we can totally delete it. To guide you further, let's follow the steps below:

1. Go to the Banking on the left panel.
2. Within the For Review tab, tick duplicate expense next to the Dates column. 
3. Click the Batch actions drop-down arrow.
4. Then, select Exclude. 
5. Once done, go to the Excluded tab. Find that expense and put a check-mark next on it.
6. Click the Batch action drop-down arrow one more time.
7. Hit the Delete option. 

Sample screenshots:

Also, you can always visit these articles where additional information about handling transactions in QBO can be found:

• Categorising and matching online bank transactions.
• What to do if QuickBooks matches the wrong transaction.

I'm always here to assist you more if you need further help in managing transactions in QuickBooks. It'd be my pleasure to guide you. Take care.