Stamp duty

I have read RERA act and from that i have one confusion for which i need clarification. According to section 13 it says as follows ." (1) A promoter shall not accept a sum more than ten per cent of the cost of the apartment, plot, or building as the case may be, as an advance payment or an application fee, from a person without first entering into a written agreement for sale with such person and register the said agreement for sale, under any law for the time being in force." So if i am going to a developer to book an under construction flat,do i need to pay 10 % of flat price as an advance+stamp duty as per local percentage+registration fee all togather and leave it to developer to deal with stamp duty payment on my behalf or i just need to oay 10% of booking fee and when the certificate of occupancy would be issued and the flat is ready for me to move in at that time i should pay stamp duty while getting the title transferred to my name? And one more question the said text from act says that 10% of booking should be paid while registering agreement of sales so how does it work? Where do i need to go for that? By registering agreement of sale aren't we transferring the title of underconstruction flat therefore need to pay stamp duty while booking a flat? I am from Ahmedabad juat in case if you need to know local law to assist me with this.