How do we avoid posting 2 transactions to the same reference number?

 Hi there, @jackie@sjlabels.

Let me answer your queries in QuickBooks Online (QBO).

The program automatically changes the invoice number each time a user alters it. Once you set your numbering preference, the system will follow the next number of the recent invoice. If you've used it twice, QuickBooks will automatically prompt you that the number has been used.

In case you've used it twice, QuickBooks will automatically prompt you that the number is already been used. 

To avoid duplicates, you can turn off the Custom transaction numbers feature. This will help prevent accidental changes to your numbering. Turning this off only means the Invoice no. field is hidden from your sales form but not when printing. 

Here's how:

1. Click the Gear icon on the toolbar and select Account and settings.
2. Go to Sales, then select anywhere within the Sales form content section.
3. Turn off Custom transaction numbers.
4. Click Save and select Done.

For additional reference, check out this article: How to use the Custom transaction numbers feature to set your own numbering preference.

Feel free to add a reply if you have further QuickBooks-related concerns. I'll be here to help you out, jackie@sjlabels. Have a good one!