Check if item is already in cart, if yes, then redirect user to checkout without adding product

Are you selling digital products and you are looking for a way to stop duplicate sale of the same product? In this tutorial you will learn how to maneuver on this. We are going to add the feature with:

1. User clicks add to cart button.
2. Function checks if the item is already in the cart.
3. If yes: Redirect user to checkout page without adding the product to cart (so the quantity doesn’t change, it stays at 1).
4. If no: Redirect to checkout and add product to cart (quantity goes from 0 to 1).