QuickBooks Online (QBO) will prompt you if you try to create an invoice with a duplicate transaction number, Nick. Please refer to the snapshot below for reference.

When you start creating a sales transaction, the application automatically assigns a number or letter sequence you set as an identifier. To avoid errors in the numbering, it’s best not to frequently change the assigned sequence.
If you have duplicate numbers on your invoices, you can edit them. However, before doing so, you need to enable custom transaction numbers for sales forms to set your own numbering or correct duplicates.
Here’s how:
- Go to the Gear icon and select Account and Settings.
- Click the Sales tab.
- In the Sales form content section, toggle on Custom transaction numbers.
- Click Save, then Done.

With this feature enabled, you can assign new transaction numbers or letters with a custom sequence whenever you need to start a new format.
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