I posted this elsewhere:
The only way I found to prevent this, or at least try to, is to place WP-eStore in a text widget and display a text block
“PLEASE NOTE: Reloading or refreshing a page will result in the same item being added to the cart twice. We recommend you use the website navigation menu ONLY.”
Not the perfect solution by any means but refunds are a pain in the butt.
If anybody finds another solution please post it.