Does anyone know why QB creates duplicate invoice numbers?

Hi there, groffefinance.

Thank you for visiting the QuickBooks Community. I'll be sharing details on how invoices number work in QuickBooks Online. Let's also perform steps to avoid having duplicate transaction numbers.

Please know that QuickBooks offers Custom transaction numbers feature to set your own numbering preferences. This element is activated by default. It could be there's a user in your QBO that manually change the invoice number format to 11994(1), and that's the reason the next invoice was 11994 (2).

If you don't want to use it, I suggest updating the invoice number format when creating an invoice such as 11994 or 11995. This way, the system will generate the correct number sequence. 

Additionally, if you wish to turn off the said feature above, you may open this article to see the steps: Add custom transaction numbers to sales forms in QuickBooks Online.

Lastly, I'm adding this article to see details on how the Sales page gives you a great at-a-glance view of the status of your sales transactions: View sales transactions in QuickBooks Online.

Please feel free to leave a comment below if you have other concerns about managing invoices in QBO. I'm always ready to help. Take care, and have a great day, groffefinance-gm.