How to get duplicate will

Normally registration department not easily given. but you can submit your father's death certificate along with the legalheirs certificate with an affidavit . once the registrar satisfy your submissions then they can issue it.

Apply for certified copy from the Sub-Registrar where the document is registered.

You should consult and engage a local prudent lawyer for appreciation of facts / documents, professional advise and necessary proceeding. 

Apply with a legal heir certificate, Photo ID, and death certificate and you can get a certified copy within a week.  At some places except for a copy of such an application, no documents are insisted for providing a copy of the will.  But this is a simple routine issue and you can get a certified copy within a week..  In view of your disabilities entrust this work to an advocate clerk who can get it for a sum of Rs.200/- + Rs.20 stamp paper payable to sub registrar.

Avoid making many posts with a similar query.

You can get the certified copy of the registration document of the Weill held in the sub registrar's office.

You can take the help of a document writer to procure the same.