1. It seems that you did not carry due diligence before the purchase of the property.
2. If the property purchased by you has been previously sold to another person then he alone could have sold it to you. Sale of the property by any one other than the owner is illegal.
3. The actual owner of the site can file a case for house trespass against you. Furthermore, he can file a case for your eviction in the court to evict you.
4. The legal recourse available to you is to move to court and file a case for cheating against the seller and also sue him for recovery of the amount paid by you towards the consideration of the site.
5. Your sale deed has no legal validity if it was not executed by the owner.