Imports under high sea sales agreement

Dear Members

We import  from USA under High Sea Sales Agreement from the Pricipal vendor. Under the High Sea Sales Agreement principal Vendor's Company in India invoices us. Based on the Invoice and shipping documents, we pay customs duty and clear the consignment.- Till 30th June-2017

I would like to have clarifications on the following:-

• Can the Pricipal Vendor Company charge IGST on the imports meant for us?
• If Principle Vendor Company can charge IGST, then I will be paying again customs duty and IGST

Awaiting your clarifications

Thanks