What you have done is considered a “standard practice” by many websites, especially banks. Some sites go so far as to disable the browser back button; but this is considered intrusive, because it makes the user think you are “doing something” to their computer.
If you are interested in learning more about usability standards and practices, the following is suggested:
W3C Web Content Accessibility Guidelines 1.0
Possible Workaround
Use the following option of eStore as an workaround if you think this will be an issue for you:
1) Create a “Checkout” page and place the shopping cart shortcode in this page.
2) Go to eStore settings and specify the URL of the page created in step 1 in the “Checkout Page” field.
3) Then tick the “Automatic redirection to checkout page” checkbox and save the settings.
Now after someone adds an item to the cart they will get redirected to the checkout page automatically and if they hit the “browser refresh” button it won’t add the item again.