Force a new line item on cart
To force to create a line_item for every same product added to the cart you just need to uncheck the option "Attempt to combine like products on the same line item in the cart." in the Add to cart form settings  of your product reference in your product display content type.

In other words. You have to change the content type that you use to show your product, you had added a product reference to this content type. Then you just has to change in the "Manage display" of your content type, the settings of the "Add to cart Form" of the product reference field, and you have to uncheck "Attempt to combine like products on the same line item in the cart."

This is a screenshot of the manage display page: