Importing Invoices - Preventing Duplicate Invoice Imports

I appreciate what you've done so far, AdrianFL. Yes, there is. You can turn on the Custom transaction numbers option in the Account and Settings to detect duplicate invoice numbers from importing to QuickBooks Online (QBO). I'll guide you through the steps below.
 

1. Go to the Gear icon and select Account and Settings.
2. From the Sales tab, go to the Sales form content section.
3. Turn on the Custom transaction numbers.
4. Click Save and Done.
   

You can also check out these articles to learn more about the use of custom transaction numbers and importing invoices in QuickBooks:
 

• Add custom transaction numbers to sales forms in QuickBooks Online
• Import multiple invoices at once in QuickBooks Online
   

Moreover, please check this handy article for future reference in recording customer payments: Record invoice payments in QuickBooks Online.

Mention me in the comments if you need further clarification on importing your invoices without duplicates in QuickBooks. I'm readily available to assist you.

Many thanks! I will be trying it out soon.

Additinal question I have, but maybe it is better to start as new thread if you suggest it.
Do you have any logic for taxation developed to cover Federal Excise Taxes (FET) taxes for aviation business?
The Federal Excise Tax (FET) in the U.S. isn't always a simple percentage applied to the total fare.
It has specific rules regarding what portions of the flight are taxable, and certain segments may be excluded from your overall trip charge.

Do you have any customers implementing FET within Quickbooks and what could be possible suggestions to implement it?