A invoice with the same invoice number and vendor coding already exist's in Sage 100 and is unposted.  

You should confirm that the invoice is in fact a duplicate invoice. If you find the invoice in Sage 100 is the same invoice, you can simply "Mark the Invoice as Exported" in Beanworks. Since the data is already in Sage, you don't need to export it again.

It's possible that the invoice is actually a different invoice, this sometimes happens with vendors that are poor at using unique invoice numbers.

In this scenario you should reset the invoice in Beanworks and amend the invoice number to something unique. Once you export and sync the invoice again the invoice will populate in Sage as it no longer views it as a duplicate.