In case my father executes a Registered Will now and subsequently any of his sons or grandsons who have his signed blank papers misuse those papers to prepare a subsequent Un-registered Will without the knowledge of my father, then which Will would prevail, Registered or Un-registered.?

I am assuming that it would not be easy for me to prove that unregistered will is fabricated one because blank papers are signed by my father. How can I safeguard my interests under the Registered Will or any other manner or how can we make the use of signed blank papers without the knowledge of my father null & void.?