We have seen before that in some cases when using our shopify module that when adding to cart from inside our personalisation iframe that 2 items are added to the cart.

This is because of a setting in shopify which can be easily disabled

When on the Store you need to navigate to the Theme and "Customize" the live theme. At the top on this page select "Theme Settings" and there should be some setting regarding staying on the product page when adding to cart. If you disable this and save then the problem should be solved.