I think its general common sense, if you passed a course you never ever want to touch it again, let alone redo it once its done.
I don't know if there is a formal rule regarding that, but I think the student centre once they figure out what is going on, will just point and laugh. I mean, think about it.
If you fail, and redo the subject that is different that passing and re-doing the subject. (In case I drifted off topic)
Saying to a potential employer, look I got a fail, ain't going to make a good impression no matter how talented your later year marks might be. Of course you could use the I got better. But of course someone similar to you could say my marks went from P to HD, not F to HD. Relatively you'd still look shitter and FL is like a mini-stigma, a P is a P, its a pass, a F is a F, a FAIL. It shows you couldn't (for whatever reason) do xyz course.
In any rate, I'd suggest don't bleedingly point out your fail in your transcript, unless they ask about it.
And remember, even when you re-do the failed unit the first mark is still only taken in the calculation of your wam. So even if you get 90 second time round, it means nothing.
That kind of brings back to the first point, if there is no formal rule, there is a rule of first mark sticks in the calculations. So if they let you retake a course you passed (say they just wanted your money and didn't care about your odd demands) the second mark would be bubkus in any rate.