What happens if an invoice number already exists in my integrated accounts package?

Nothing. The system will record an error in the 'Feedback' grid of the [Accounts | Accounts Sync | Export] screen and the invoice will not export.

You then have two options;

1. Green select the invoice in the top 'Pending Export' grid, click the orange Grid Actions button and select Mark as Exported. This action will prevent KCC making any further attempt to export the invoice - and then manually create the invoice in your accounts package (if it does not already exist). 

2. Edit the existing Invoice in Xero so the numbers are not longer matching and then retry the export.