I'm here to ensure that you'll be able to exclude your duplicate transactions in QuickBooks Online, @GFWS.
Being unable to exclude one of the duplicate bank transactions in QuickBooks Online can be caused by a partially matched entry. To get this issue sorted out, I recommend unmatching the transaction from the entry that you have in QBO.
To do that:
- Go to the Banking menu.
- Refer to the Banking tab.
- Select tile for the bank or credit card account you downloaded the transaction from.
- Select the Categorized tab.
- Find the transaction you want to unmatch.
- Select the blue transaction link in the Added or Matched column to see the matched transaction.
- Press the Undo from the Action column.

For additional information about unmatching bank transactions in QBO, please check out this link: Learn how to undo or unmatch transactions downloaded into QuickBooks Online.
Once done, you can proceed with excluding the bank entries.
Here's how:
- Go to Banking menu on the left panel.
- Choose Banking tab.
- Check the box for beside the transactions you want to exclude.
- Click the Batch actions drop-down and choose Exclude Selected.

I've got here a reference that you can bookmark to guide you in excluding bank transactions: Exclude a bank transaction from your expenses in QuickBooks Online.
Drop me a reply if you have any follow-up questions about excluding transactions. I'm always happy to help.