I've been contemplating this too.
You're under no obligation to repeat it unless its an essential component of your course (usually applies for courses where you have little choice in your subjects: law, engineering, ect). Second situation is if you need it as a prerequisite but by judging by the fact its an elective i doubt this applies to you.
If you fail 1 unit in one semester you might get a warning email that encourages you to speak to someone in your faculty but thats about it. If you fail more than one in a semester or fail a unit in semester 1 and a unit in semester 2 disciplinary procedures might apply. I'd do my best to avoid it because to be honest they sound scary as shit. Don't mean to scare you though but i think you should be aware. Failing one unit is fine, i'm sure plenty of people have failed or just barely passed plenty of units. The procedures really come into effect if you massively fail(i think its if you fail 50% of your units in one year). Still well worth knowing though. I think every student should know and at least to me i had no idea of this until i found it out myself.
If it gets really bad they can even
chuck you out permanently(exclusion). However, you're
very far away from this stage. Like i said you'd need to fail most of your course, which you aren't doing by the sounds of it, so all is well.
I'll link you the science faculty ones because thats the faculty im under. Not sure which one you're under but might want to take a peak.
http://sci.monash.edu/undergrad/unsatisfactory.html (be sure to read the two links at the bottom as well, exclusion and unsatisfactory academic progress)