This is easy enough in the Table. Assuming you have a field called "Invoice" that contains the number, just make a new filter. In the expression editor for the filter, just type 1, which will basically return all records. Then click on the Advanced tab of the filter editor, and look at the options for Duplicate handling. Specify that you want to look for duplicates of the Invoice field, (or Invoice and Supplier), and then say that you'd like to see just the duplicated records. Save this filter, and you should then get the table, filtered to just the duplicates.