I'll assist you in resolving these duplicate invoice numbers, Colonel.

Since you mentioned that the system is currently using duplicate invoice numbers from a year ago, it’s possible that the Custom transaction numbers feature was enabled. This would have allowed a user to manually enter a number that had already been used, causing the numbering sequence to reorder. To verify, you can use the Audit log to view any modifications made to your account.

Normally, QuickBooks Online (QBO) will prompt you if an invoice number has already been used to help prevent duplication. If the Custom transactions number functionality is turned on, you can manually override the invoice numbers by typing new ones in the designated field.

However, if you're unable to do so, double-check your settings and make sure it's enabled. Here's how:

1. Click the Gear icon and select Accounts and settings.
   
   
   
2. Go to the Sales tab and click the pencil icon in the Sales form content section.
   
   
3. Ensure that the Custom transaction numbers feature is toggled on.
4. If not, turn it on and click Save, then Done.

To resolve the issue with duplicates, locate all invoices with duplicate numbers and then manually change each one to a unique number that is higher than the most recent invoice number. This will restore a proper sequence and avoid confusion. I've added a screenshot below for your reference.

Please don't hesitate to drop a comment if you have additional questions.