Thanks for giving details about your invoice number issue, @Kelly_T. I'd be happy to assist you in fixing the duplicate numbers.

You can manually edit the invoice to enter the correct number. To do this, we can pull up a report to see all the list of invoices and locate those that have duplicate numbers. Before performing the steps, I suggest creating a backup copy of your file in case of data loss or damage. When done, please follow these steps:

1. Open your QuickBooks Desktop (QBDT) program and click the Reports tab.
2. Hover over to the Customers & Receivables section and choose Transaction List by Customer.
3. Then, select the Customize Report button. Set the date to All.
4. Next, go to the Filter tab and search for the Transaction Type option.
5. Choose Invoice and click OK.
6. Once done, locate the duplicates and double-click to open.
7. Then, input the correct invoice number and select Save & Close. 

After that, let's update your invoice settings to prevent having duplicate numbers in the future. Let me guide you how:

1. Navigate to the Edit tab and click Preferences.
2. Next, click the Sales & Customers menu.
3. Select Company Preferences.
4. In the Sales Forms box, check the Warn about duplicate invoice numbers option. 
5. Then, click OK.

Once finished, let's now create a test invoice to check if the number will follow the last invoice number in the report.

If you still experience the same thing, you can use the Verify and Rebuild Data Utility tool to verify and fix possible data issues. 

QBDT makes it easy to process your customer's payments. You can send them an invoice that they can pay online or manually record the payment. To know more details about this, feel free to open this article: Receive and process payments in QuickBooks Desktop.

You can count on me if you have additional concerns about having duplicate invoice numbers in QBDT. I'd be glad to help you further. Have a good one.