In Kolleno, navigate to the specific Customer Hub > Configuration
Here, you will see all contacts imported from NetSuite for your customers.
On the screenshot:
(1) If you go over the import source (here, "NetSuite"), you will see where the contact comes from such as : "invoice_08384_cust_my_field" means that it comes from the Invoice record, NetSuite internal ID 08384, and field 'cust_my_field'.
(2) The column "Invoices" shows all the invoices that contact is linked to in Kolleno.
If you have the same email address coming from multiple invoices, it may look like duplicates. But it you pay attention to the Source and Invoices column, you will see they are different.