Stamp duty payable on delayed registration- mumbai

Hi, Would be grateful if some could help me on my queries below:- Facts My father bought the current house he is living in June 1996 for an agreement value of INR 15 lakhs and also paid the appropriate stamp duty payable at that time of INT 78,700. However he and the seller did not register the sale with the registrar. The society has duly inducted him and the share certificates have also been transferred. Subsequently based on someone’s advice during the stamp duty amnesty scheme in 2008/09 he decided to complete the registration procedure and paid additional stamp duty if INR 54,800 on the revised flat value of INR 21.85 lakhs. But unfortunately due some unexpected events he and the seller could not go for the registration this time too. Queries 1. Is registration of a property compulsory? Are the buyers rights who has a duty executed agreement in his favour (appropriately stamped) in anyway diluted for lack of registration? 2. In case he decides to sell the house, will the sale process be hampered in any way due to non-registration of the current agreement? Will it make difference in case the house goes for redevelopment? 3. Assume he was to go for registration now does he have pay additional stamp duty again at the current assessed value of the house? Regards Prashant