Variable Option Issue

When I add variables to products in the store - ie. Medium, Large, etc. - when a user goes to add the product into the shopping cart they are directed to the attached screenshot. When you try and refresh the page, it just adds another to the quantity of the specific product. Other than this issue, the shop is working fine.

I am using all of the latest updates. My theme is Studio using Classic presentation.

Any help would be greatly appreciated!