Why does QB put the word duplicate after every invoice number? It's getting really annoying having to edit every invoice before I send it.

Hello there, damianpeace.

The most common display issues might be caused by too many cache files in the browser. 
 

To help fix this, I recommend accessing your QBO account via a private browser. It's a great way of isolating the issue since it doesn’t store data in the cache. Here are the following keyboard shortcuts: 
 

• Google Chrome: CTRL + Shift + N
• Safari: Command + Shift + N 
• Mozilla Firefox: CTRL + Shift + P 

From there, you can create a test invoice to see if there's still a word duplicate. If it works, you can go to your regular browser and clear its cache. This deletes the stored cache files and get a clean slate in the browser. If not, you can use other supported browsers. 

If you need help customising your invoice, you can check this article: Customise invoices, quotes, and sales receipts in QuickBooks Online. It provides steps  on how you can personalise and add specific info to the form.

Feel free to get back here if you have other concerns in managing your invoice in QuickBooks. I'm always here if you need help @damianpeace.