How can I delete duplicate expense transactions when two different bank accounts are involved and one account has been deleted already?

I can help you remove those duplicate transactions, @brianadams1976-a. 

The best way to remove those duplicate transactions is to exclude transactions from the active online banking account.

You can only delete the transactions from the deleted bank account by reactivating the online banking connection, removing the transactions, and then deactivating the bank account again. Check out this article for more information: Disconnect accounts connected to online banking in QuickBooks Online.

To exclude bank transactions:

1. Go back to your Banking page and click the For Review tab.
2. Select the checkboxes of the transactions to exclude.
3. From the Batch actions drop-down, click the Exclude Selected.

The transaction moves to the Excluded tab of the Banking page. You can delete the transactions from there. 

Refer to this article for more information about excluding bank transactions: Exclude a bank transaction you downloaded into QuickBooks Online.

From here, you may consider reconciling your accounts to ensure your books are accurate and there aren't any duplicate transactions: Reconcile an account in QuickBooks Online

If you have other questions about adding your bank transactions, I'm happy to answer them for you. Just post it here as a reply and I'll take a look at it. Have a good one!