Well, this feature cannot be generic. You have to handle such things in your ...

Well, this feature cannot be generic. You have to handle such things in your project itself. The add to cart button is not rendered from this module. This module only adds an AJAX behavior to add to cart button.

I guess the fix will be - inside the preprocess function of add to cart form (or form alter) you can check for the presence of product in current user's order, and then alter the add to cart button according to it.