Land sold to multiple people

Hi, We have bought 10000 sq feet farm land in Bangalore.We have been running small office in this property for the past 5 years.It was on lease initially , we bought this land almost an year ago.We have Khata and all the documents for the property. Last week a local guy has approached us mentioning that the seller had sold 2000 sqft to him from our land.It happened before 2 years ago.He also has EC with him. Our registration document has land in Guntas , this guy has got land with a plot number mentined.How can we sort this issue.If we file a case who can acquire the land. Thank u