Does GST is calculated on previous added GST for the same product GST will be on transaction value

Your understanding the GST is incomplete...

Person A charged GST of Rs.18/- to you.

Now when you sell the item and say you charge RS. 27 to your customer, you have to pay Rs. 9/- to Government, as Rs. 18 was already paid earlier, which is known as ITC (Input Tax Credit) for you.

The final burden of all the tax is on the consumer, as GST is consumption tax. Rs 18 which you paid to your supplier has already been paid to the government. As far as your supplier is concerned, you are the "consumer". But if you add value and sell it further, you are no longer the consumer. You will charge Rs 27 from your customer, but you will only pay Rs 9 to the government, as Rs 18 has already been paid. This Rs 18 will show against your GST number as ITC (input Tax Credit) and your liability will only be Rs 9.

Now suppose the person you sell it to is not a consumer but also a business, then you will enter their GST number in your returns and Rs 27 will show as their ITC. So if they sell it further, they will only pay the amount which is over Rs 27 to the government.

This is how GST works. You only pay tax on the value you added if you sell it further (or use material and make another product out of it and sell it). If you are the final consumer, you pay the whole tax which occurred along the supply chain of the product, right from raw materials.