How do I unlink an outside back account that was duplicated?

Hello there, @harsenshandyman-. I can help you disconnect the linked bank account in QuickBooks Online (QBO).

You can edit the account information of the duplicate bank account in the Banking page and disconnect it from there.

Before that, ensure that the duplicate account doesn't have any transactions you need to categorize because you will no longer see those transactions in the For review section or in the Chart of Accounts.

Now, here's how to disconnect the bank account in the Banking page:

1.  Go to the Banking menu and select Banking.
2. Locate the duplicate bank and click Edit account info.
3. Tick the box beside Disconnect this account on save.
4. Select Save and Close.

For more details on how to handle your linked bank accounts, you can check out this article: Disconnect accounts connected to online banking in QuickBooks Online.

Moreover, you can read this reference you can use on the future: Learn the reconcile workflow in QuickBooks. It contains tips and techniques on how to account your business financials.

Let me know in the comments below if you need further help about disconnecting your bank account. I'll get back to you as soon as I can.