woocommerce add to cart with redirect doubling quantity

I’m trying to implement a woocommerce add to cart url that redirects the user to another page (so I can try to upsell another product) before going to the cart.

I’m doing it through URLs because it’s more of a sales page type of sale and not the type of site where you sell things on a grid (like a normal woocommerce site)

The technique I’m using can be found in this article:

https://proserveweb.com/create-woocommerce-link-add-product-cart-redirect-checkout/

Essentially you create a link like this:

https://yourproducturl.com/change-this/?add-to-cart=ID

You simple switch out “change-this” for the page slug you want to redirect to…the PROBLEM is that when you finally do get to the cart the product has been added twice. See screenshot:

https://screencast.com/t/fBBgPZ1sVjB

Not sure why this is happening but obviously it kills this great little trick ??

Any help would be REALLY appreciated.