An invoice with the same invoice number and vendor coding already exists 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 SageAPA. 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 SageAPA 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.
A little advise:
If you have a few worst offender vendors when it comes to invoice numbering or don't provide an invoice number at all. Make a practice of changing the invoice number in the coding phase.
I've a vendor that doesn't send me an invoice number but rather a customer number which is the same on every invoice I receive from them. I could use the customer number plus the date when coding the invoice number to make it unique i.e. customer number: 200392 becomes invoice number: 200392-MMYY. This way you'll cut down on the amount of possible sync errors.
It's important if you introduce this policy that your entire coding team are following the same process. If not strictly followed a real duplicate could slip through the cracks.