From my understanding, you can only count bio once and the highest Bio score will count to your ATAR. So if you perform better next year (as an example), you will overwrite your score from this year.
Since you are dropping one of your six subjects and not doing the U3/4 for it (to redo bio), you will only have 5 subjects that will count to your ATAR i.e. top 4 subjects (which will include your English subject no matter what) + 10% from 5th subject. This means you’ll lose the potential for another 5 ATAR “points” compared to doing a 6th subject. This can be a risky move as (1) you cannot guarantee that you will do better the next time and (2) you might be able to do better overall by doing a 6th subject that may move bio to your 10% subjects. However, if you’re willing to put in the effort and raise the bio score significantly, it can reap rewards. Here’s an old thread about repeating a subject.
