Spurious display of add to cart popup

The expected behavior is to have a message displayed when an item is added to the cart, e.g.:
"Product successfully added to the cart"

This works when clicking "Add to cart" from a categories listing screen, but not from the product details screen.
Pressing the browser's "back" button from product detail display to category display causes the message to be displayed again, as does refreshing the screen (message is displayed again though an additional item isn't actually added to the cart).

Hello,
Can I have a link to your web site then I'll maybe see what the problem is ?
thank you.

Thanks Mohamed. I've concluded this was an issue with my browser, was strange for days and now I can't recreate the issue.