If you're reading this, chances are you received the following error:
"Duplicate Document Number Error: You must specify a different number. This number has already been used."
This means the invoice number that is trying to be processed already exists in Quickbooks. We'll want to change a few settings to get around this.
In Quickbooks, head over to Settings followed by Account and Settings >> Sales.
Ensure Custom Transaction Numbers is OFF.
3. Now click on the Advanced tab on the left.
4. Under "Other Preferences" ensure "Warn if duplicate check/bill number is used" is also set to OFF.
Now you'll be good to go - no more error!