Duplicate document with seller without intimating to buyer...

I have purchased a property (constructed flat - 2nd sales) 7 years back. Built up is 675 Sq.ft & UDS 302 Sq.ft Document no : xxxx/20xx.With Survey no :xxx/xx (document registered in My name ). Eventually i have checked the survey number in online registration portal & came to know the seller has other document with him in same name as Built up 675 & UDS 300 Document no : xxxx/20xx.With same Survey no & he has not revealed the information to me at the time of selling. On asking about this,he told that is additional UDS of 300 Sq.ft registration (i.e Total Built UP area is 675 Sq.ft ,UDS-602) & he will transfer the same to me immediately.But after several follow ups,he told that he has missed that particular original document & later informed he has applied for duplicate document request & will make registration procedure once he got back the duplicate copy of documents from a specific register Office.Kindly advice me how to go about this registration process. Also clarify me that this seller cheated me for last 7 years without giving full document even after my possession.