Hi Souvik
Deduction for stamp duty paid shall be available in the year in which such amount is paid.
There will be no deduction allowed for principal repayments when the property is under construction.
Interest repayments upto 31 March 2019 (provided construction is completed in may 2019) shall be provided as deduction in 5 equal installments starting from FY 19-20 I.e. year of completion.