When we create a new invoice, we get the message 'You've used it twice, there's already an invoice with xxxx'. But we didn't create any invoices with the same number.

I appreciate that you tried to fix your invoicing concerns in QuickBooks Online (QBO) by troubleshooting steps to clear the caches and cooking using another computer. Let's resolve this by advanced search using the invoice number in concern, Order6.

You can find an invoice by its number using the default Classic Search feature or the Transactions Search feature. To search for an invoice by number, utilize the Transaction Search or Advanced Search options. If this persists, recommend reaching out to our Support Team for assistance, as they can look into the issue. Here's how:

1. Click the Magnifying icon on the top-right part of the page.
2. Select the Go to Transactions text button.
3. Enter the invoice number in the Reference number field and press Enter on your keyboard.

You can explore this article for more details about the Advanced Search feature: Search for transactions in QuickBooks Online.

Here's how to contact our Support Team:

1. Login to your QuickBooks, and click the Help icon.
2. Then select QuickBooks Help/Contact Us.
3. Give a brief description of your issue, then select Continue.
4. Sign in to your Intuit account select Continue and then Continue with my account.
5. We'll email you a single-use code. Enter your code and select Continue.
6. Select to chat with us or Have us call you.

Our support services are available Monday to Friday, 6 AM to 6 PM Pacific Time, and Saturday from 6 AM to 3 PM Pacific Time.

After that, you can also consider checking this article to find, review, and edit your transactions in the account registers.

If you have questions about invoicing or any QBO-related concerns, I'm here and happy to help. Have a great day!