Property sold to multiple buyers

As per law,  immovable property can transferrred by registered deed only. Thie deed should be registred in the office of sub regsitrar and stamp duty be paid. Since you have the registered deed in your favour, you are the real owner.  

Sale deed can never be executed via registered agreement , it can be only registered in the office of sub-registrar where both the parties are present and 2 witnesses. 

If there is only registered agreement you have, then first file a criminal complaint against the builder of cheating, fraud at your nearest police station, after F.I.R lodged file a civil case of suit for recovery of money.

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If you have executed the sale deed then file a criminal complaint against the builder at your nearest police station lodged the f.i.r  after then file a civil case of Specific Performance.

The second person will not have any claim over the property. Your builder has commited offence for which you can file complaint in local police station and also approach consumer forum. Please appoint advocate.