Land sold two times in unauthorised colony

We bought a plot in dwarka in an unauthorized colony nr dwarka sec 15 in 2012. Some other property dealer sold the same plot to another party now in 2015. We have registered GPA, sale deed and will certificate. They also have the same chain as we have with the last person as different. Their property dealer and last person are missing since the day we found their board and lock in our plot. Our dealer and the last seller are present with us. After we found their lock on our gate(there was a four sided wall and we had put our gate there) we called them and went to both the dealers and found theirs missing. Then we again put our lock. Upon advice from dealer we built a small room and bathroom there. Meanwhile they complained to police and they told to stop construction for few days. Now they have registered a case. Prior to that we showed our documents in police station and police took statement also. Now what should we do? Kindly help. We spent entire life earnings to buy that plot. In stress also.