Dear QB how or where can we receive a quote to upgrade our qb package to enable us to receive alerts when invoice numbers are duplicated?

It's good to see you here in the Community space, @jan87.

I'll be happy to share information on how QuickBooks Online (QBO) alert you when having a duplicate invoice number.

In QBO, there's no need for you to upgrade your subscription to receive alerts when having a duplicate invoice number. Since we have a setting where you can customise transaction numbers and let you view and change them. By turning it on, the system prompts you that there's already an invoice number assigned to previous invoices. 

To activate this feature, here's what you'll need to do:

1. On your QBO account, go to the Gear icon at the top.
2. Select Account and settings.
3. In the left menu, click Sales.
4. Go to the Sales form content section and turn on Custom transaction numbers.
5. Click Save, then Done. 

That should do it, @jan87. If you'd want to personalise and add specific info to your sales forms in the future, you can visit this article for a guide: Customise invoices, quotes, and sales receipts in QuickBooks Online.

Please keep me posted if you have any questions about QuickBooks. I'm always here to help.