Pantaloons Fashion & Retail — charging of gst twice during sale period

Address:

Senapati Bapat Road, Pune, Pune, Maharashtra, 411016

Yesterday I shopped from your Senapati Bapat Road Showroom, in Pune for ₹ 4869.48/-
I was told that an extra 5 per cent GST will be charged over and above the discounted price as the sale price did not include GST.
However, my bill (attached) already showed me a GST of 12 percent. So why is pantaloons charging an overall GST of 12 + 5 percent?
The calculation of the bill was as follows:-
MRP ( which already includes 12 percent GST) - 20 percent discount + 5 percent GST.
The same can be verified from the bill attached.
So my query is why is GST being charged twice. Either claim upfront that the discount is only 15 percent or atleast show the calculation in the bill as to why the extra 5 percent GST is being charged.

Warm Regards,
Deven Savant