This problem is bigger than JavaScript and dynamic applications. Even decades ago you'd see warnings to "please only click submit once", because most users don'

This problem is bigger than JavaScript and dynamic applications. Even decades ago you'd see warnings to "please only click submit once", because most users don't look at the browser activity state to see if it is actually doing something (regardless of it being an spa or classic pages). There needs to be two kinds of forms, one that asks if you really want to re-submit, and another that allows infinite submission. The backend should handle the problem too, but this would go a long way toward a better default UX.