Sale deed of property

I had purchased a ready built / ready to move in flat from a reputed builder in Rudrapur, Uttarakhand. The property was registered (Article 23 Conveyance) at the Sub Registrar's Office, Rudrapur in my name on 11 July, 2017. Though More than 10 months have now lapsed, the builder has not yet handed over the possession of the flat, inspite of repeated requests. Recently i've come to know that the builder had earlier sold the same flat to another buyer. But no registry was done in the former's name. Now the builder has sold the same flat to me and completed all legal formalities including registration. But the builder is not handing over the possession of the flat to me. What should I do?