Prevent form reload (login, shopping cart... etc).

Today I was looking through my draft posts and saw this one about preventing form reload, and this is the note I took at the time: "set session variable to empty at form load, set it to something at successful submit, if session variable set when submit clicked, throw error" - well, yes, that was the first thought about it I found on the net, however, after I continued to look, I found that there is a better, more efficient way to do this: