If you receive the error "Duplicate Document Number Error: You must specify a different number. This number has already been used." or similar when trying to push a booking/payment across to your QBO account.
You can easily fix it by following these steps:
1. Open your QBO and navigate to Settings >> Account and Settings >> Sales
2. Check that 'Custom transaction numbers' is turned OFF
If its on you can turn it off by clicking the edit button, and then clicking the 'Custom transaction numbers' checkbox.
3. Then go back to QBO Settings >> Account and Settings >> Advanced
4. Check that 'Warn if duplicate check/bill number is used' is also turned OFF, you can turn this off by clicking the edit button and clicking the corresponding checkbox.