Drupal 7 Commerce Add to Cart button allows for multiple clicks – how do I stop this from happening?

This issue appears to be specific to recent versions of Google Chrome (both PC and Mac) and Safari on Mac and Edge on PC.

You can see the issue here: https://bookstores.umn.edu

Multiple clicks on the Add to Cart button will add more products to the cart (using latest versions of Chrome, Safari and Edge). Firefox appears to be OK.