Confirmed in J and waitlisted for F is perfectly fine
Nothing at all wrong with that.
What could happen here is that if the flight is looking full and SQ needs seats (or perhaps they're trying to clear waitlists) is that they'll review the flight, see you are booked MEL-LHR in both J and F, and cancel one of them. Hopefully not before contacting you, but you might miss their call.
Taking one award seat in J, and another award seat in F when you can't possibly use both of them, is potentially stopping another passenger from using it.
Some folk may think 'tough luck'. Others may be aware how hard it can be to find seats and be mindful of that.
On something like this I'd probably call KrisFlyer, get them to review your bookings, and ask them to send a chaser to try and clear the outstanding waitlist for the one passenger in F. Then they can sort out the seats you won't be needing.