Purchased site is already sold to someone else. Requesting help with this.

Hello sir, I have purchased a site in Harohalli Bangalorem, the layout name is Alpha RJ layout. This is 40 acers layout and the first phase is approved by BMRDA. I registered one of the BMRDA the site in 2011 by paying the complete amount and an absolute sale deed was executed and hoskote registration office. But recently couple of years ago, we the land owners discovered that the some of the survey numbers are already sold to someone another person, when we checked when it was done, we found that it was done in 2010. So now we are not sure what to do. How can the government body give permission to form the sites for already sold land? Builder has cheated us. When we ask builder he says that he registered it to his name to get the credit for forming the layout and now he has incurred losses and not able to pay back the money to recover the land back and asking for time from last three to four years, but nothing is happening. Can you please help in this regard? how can we approach this problem and how to get our site or the paid money back from the builder? Please help in this regard. Thank you!