my client was reconnecting his bank feed, and somehow messed it up and created a duplicated bank account instead, what is the best way to deal with this please?

I agree that the original account should be retained. However, since the duplicate account is linked to online banking, we'll want to disconnect the bank feed and make that account inactive before reconnecting the bank to the original account.

Before proceeding, please note that when reconnecting a bank to QuickBooks Online (QBO), you'll want to do it in the Chart of accounts (COA) to prevent the creation of a new account.

First, check if any unreviewed transactions are sitting in the Pending tab for that duplicate account. If these are new transactions that have not been added and aren't in the original account, you can leave them as they are. But if it's already posted in the COA, Undo them to move them back to pending.

Next, let's disconnect the bank feed from the duplicate account.

1. Navigate to All apps, then Accounting, and select Bank transactions.
2. Click on the account tile and select the pencil icon.
3. Pick Edit account details.
4. Check the Disconnect this account on save.  
5. Once done, hit Save.

After that, inactivate the duplicate account in the COA:

1. Go to the Gear icon, then Chart of accounts.
2. Look for the duplicate account or enter its name in the search box.
3. Under the ACTION column, click the Account history drop-down.
4. Hit Make inactive.

Now, here’s how you can reconnect it to the original account in your COA:

1. In the Chart of accounts page, locate the original bank and click the Account history drop-down.
2. Select Link account.
3. Follow the instructions on your screen to reestablish the connection.

Once completed, it will automatically download transactions for your review and categorization.

I'm always here if you have further questions. It's my pleasure to help.