You can see all the duplicates in each part of the workflow by creating a custom filter for duplicates.
To create a 'Duplicates' filter
In any part of the workflow of the Invoices module, select 'Add Filter'.
Select the drop down bar and scroll until you find 'Duplicates'.
You can rename the filter by clicking on the name of the filter ('Acme: New Filter').
Check the box and select 'Search'.
This filter is not saved for every step of the work flow (In Progress, Approvals, Export, etc.). Turn it on by selecting the circle next to the filter name. An empty circle means that the filter had not been applied.