Having both an open and historical invoice with the same document number can occur when an invoice is open at the same time the batch is being posted. See the example image below:

If you believe that you have a duplicated doc number within SalesPad/Dynamics GP, you will need to contact your GP Partner for assistance in correcting the records within Dynamics GP. SalesPad Support may be able to provide you with a SQL query that will help you identify the records causing the issue however this will still need to be updated in Dynamics GP.