Hi Saurabh,
Thanks for asking your question. Looking at your logic and domain model, I would expect that you are using a ‘set’ instead of an ‘add’ action inside your Change Product action. Could this be the case?
Your Product_Cart association is many-to-one, meaning that a product can only be in one cart. There is also a Cart_Product association which is many-to-many, which is better, but you don’t use that one in the microflow.