Regarding plot sold on me duplicate registry

I bought a plot which was sold to me by providing duplicate registry ,on being asked owner told me it was lost or theft and therefore they will not be able to provide the original registry of the plot, they gave me the NCR complaint copy of the same and also shapath patr on stamp that this property belong to them and they are the sole owner of this property, I made the complete payment to them and got registered this property in my name but after few months when i visited the township society office i came to know that it was sold by their father before his death to someone else on original registry two three years back . I shocked to saw their original documents and i then contacted with the persons from whom i bought and paid money .They agreed that they have sold this property which was already sold by their father before his death ,and committed me that they will return the money back to me with interest, all this incident happened one and half year back and till date they are extending the payment time , now when i am ask for the money they are delaying the dates and saying you have to wait this is the only option you have or do or go wherever you want. Please suggest me what should i do