Yeah as has been said you won't be able to do a subject you have already passed.
Although, when I originally transferred courses at the beginning of the year, I told the person who was on the computer enrolling me what subjects I wanted to do (one of which was a subject I had already did, just wanted to see if I could get a better mark in it), and she enrolled me in all of the subjects without consideration of any prerequisites or prohibitions. Later on though I withdrew from all my Semester 2 subjects to pick entirely different ones, and I couldn't re-enrol in that subject I had already done through myUni.
I don't know, maybe I just got lucky with a lazy enroller
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