getting registration request for a property which was sold 15 years back

We sold a property during 2005 to a buyer by a specific power of attorney and the same was registered by the sub registar. Now I am getting calls from the same Buyer to sign the sale deed by the existing legal heirs. Is it normal or was the property not registered earlier? Is the specific power has been null and void after the lapse period of 15 years after the 2 of the legal heirs have expired. Can some one explain the reason behind why they approach me after 15 years?