Thank you for your reply nerdmmb, I think I might try it then drop a subject if it's too hard.
Yes Ned Nerb I'm doing a total of seven subjects. I had to do English language, methods and biology and the others all sounded good so I decided to do all of them, I don't care if one doesn't contribute to my atar and I wanted to repeat psychology to get a better score 
In that case, it seems your decision to repeat Psych can only be for reasons related to pride and is not related to what's best for your ATAR. Indeed, it very well could be detrimental to your ATAR: what if you now have two exams on one day? What if you do a few less practice exams per subject than you otherwise would have? What if doing Psych takes more attention that you anticipated and you start to get sick more frequently from stress and/or a bigger workload?
Personally, I don't think it makes any sense for your ATAR to repeat Psychology so I would advise against it. I mean... Let's put a numerical value to the "risk" and "reward" expected. The reward expected from Psych, best case scenario, might be an extra .5 onto your aggregate, which might even be meaningless to your ATAR. So let's call reward expected ".5" out of 10. Let's call risk, say, 3/10. I choose 3 because it's not huge. It's not 10/10 risky, but there is still some risk involved. Here, your risk is five times bigger than potential reward. If this were business, you'd take your money and run far away :p.
What score would you be aiming for anyway? To jump from a 35 to 45 may as well be a whole new subject. Anyway, I understand there may be personal reasons you want to repeat (feel proud of all your scores etc or something), but I think justifying what ultimately looks like a disadvantageous decision with it is not a good idea.
It'd be like going into $15,000 of debt for a new car when you already have a fully functional 2007 Holden Astra. Sure, it might satisfy you on a personal level to have a nice car, but ultimately, it just doesn't make sense to do it.