Legal remedies if an unknown person claims right over our property

Hello My father-in-law gifted a property to my wife in 2017 which he purchase in 1984 and build 2 storey building in 1987, now we have demolished the building and started reconstruction in same property, now a person has sent a notice that the property belongs to his grand father in revenue records. But we have all the records like mother deed, sale deed, gift deed, A Khatha Certificate, BBMP tax paid copies, water connection, electric connection , EC till date, new construction plan, Betterment charge paid. Kindly let us know how safe we are with respect to law.