Legal advice for plot case

My father and my father in law purchased a land in nalgonda in 2001 September from a farmer it was registered as GPA and sale agreement. But the same farmer registered as sale deed the same plot to others in 2002 January. But my father and my father in law don't know about this second registration. In 2011 December my father and my father in law sold the plot to a police constable, it was registered as a sale deed. After sale, the person who purchased from us is starting to torcher us. In starting he didn't asked money and solved some problems with help of us. He repeated this process for 5 years. My father was died on 2015 August. The person started torcher on me and my father in law civiourly from this March. He is harassing us for cancellation of the document and asking money for today's market price. He continuously calls me and gives me a lot of harassment. Can I put a harassment case on that person. His lawyer also harassing me. How should I solve this problem. Please help me.