>A good example are pages that take 5 seconds to load because they have to do so much, then you submit a form, and the page reload takes 5 seconds to go through but there is no UI feedback so you press the button a few more times because maybe it didn’t work? Then you get an error because the first action worked but the subsequent actions failed due to uniqueness constraints.

It's been standard practice for at least 25 years to disable the submit button once it is pressed. If you aren't doing this as a developer, you are practically begging for trouble, and it's really so easy to avoid that trouble.