Gst on underconstruction property

Hello Experts,

I have one Question regarding the GST liability on under construction property, Is there any ruling, GR for the following scenario -

"Service tax is paid on the full agreement Value to the builder prior to July 2017 and still builder is demanding for GST on the Installments paid to him post July 2017."

I am unable to understand, how can one person recover two taxes on the same amount from same person. 

Regards,

Anand