I have notices that the system duplicates the invoice numbers some times. I only realised it when receiving payment and looking up the invoice number. Any advise ?

Once the transaction was saved, QuickBooks updates its sequence based on the last saved number. The system duplicates a number if you manually type the same number into a different transaction. To fix this, we can manually edit the invoice number to the correct one.

Here's how:

1. Open your QuickBooks account.
2. Go to the invoice that has a duplicate number. 
3. In the Invoice no. box, enter the updated one. 
4. Click Save and close once done.

To avoid duplicates in the future, I suggest manually tracking your invoice numbers per transaction or using a consistent numbering system.

Please let us know what else we can help you with.