Hello- I continue to get duplicate customer invoice numbers even though "warn about duplicates." is selected in Preferences. How do I avoid the duplicate invoice numbers?

Hey there, @finance138.

Thanks for reaching out to the Community. 

I recommend checking your company file for any data integrity issues that can cause problems like duplicate invoices. To do this, you'll need to run a rebuild and verify. A rebuild will fix most data integrity issues, and a verify will scan the file for more errors. I've included the steps below to do this. 

Rebuild Data:

1. Go to the File menu.
2. Choose Utilities, then select Rebuild Data.
3. Hit OK on the QuickBooks Information window.
4. Follow the prompt to save a backup.
5. When you see Rebuild has completed, click OK

Verify Data:

1. Go to the File menu.
2. Choose Utilities, then select Verify Data.

Check out more details in Fix data damage in QuickBooks Desktop.

Please let me know if you have further questions or concerns. You can reach out to the Community at any time. Take care!