If you receive the following error: Duplicate Document Number Error : You must specify a different number. This number has already been used, this is because a transaction already exists in QuickBooks with this number.
If you want to allow QuickBooks Online to use duplicate numbers make the following changes in QuickBooks Online:
For sales transactions: Under the gear/Company Settings/Sales - turn OFF custom transaction numbers.
For bill and check transactions: Under the gear/Company Settings/Advanced and under Miscellaneous turn off Warn if duplicate option.