Welcome aboard to the Community, @benthepi, @accretive.
Right now, the ability to add a custom transaction number is only available in QuickBooks Online. For the Self-Employed version, please know that the digits automatically increment by 1 when creating the next invoice.
I've also checked here, and we haven't received any reported cases about getting duplicate numbers. I tried replicating on my test account, and everything seems fine on my end.
Considering it started happening for about 3 - 4 months already, I have to check the data on your account. Since I'm unable to review it without asking for sensitive information, I suggest reaching out to our QBO Customer Care Team.
One of them will need to take over the problem and perform an escalation whenever needed to get you back on track. Here's how you can reach them:
- In your company, go the Help icon to choose Contact Us.
- Type the issue/topic under the What can we help you with section and then hit the Let’s talk button.
- Scroll down to select the Get a callback option and then enter your contact information.
You can reach them within this time: Monday to Friday from 6:00 AM – 6:00 PM and Saturday 6:00 AM - 2:30 PM PST.
For future reference, you can visit the How to change the invoice number on sales forms article. It also provides information about assigning numbers to your sales forms automatically.
Reach out to me whenever you have additional questions or concerns. Please know the Community has your back, and I'm here ready to help you. Have a good one.
