Property sold to two different person what are the right of 1st buyer

Some months ago I booked a flat & gave 10% booking amount in the named of Mr A. I demanded his sale deed copy from property dealer, he provided me. In that registry Mr X has sold the plot to Mr A, & Mr A has started construction. Till here nothing seems wrong because I have booked a flat from Mr A. But at possession time when I need to register my flat, property dealer told me to make demand draft on some different name (some Mr Y), on asking he has given me another registry copy in which Mr X sold one fourth part of plot to Mr Y, here I was shocked how can Mr X can now sell the same property to others while he already sold the property to Mr A.